use:
txtTotalQ.Text = (txtQuantity1.Text + txtQuantity2.Text + txtQuantity3.Text + txtQuantity4.Text + txtQuantity5.Text + txtQuantity6.Text)
The + with text will be use for appending text
ie.
"A" + "B" = "AB"
However you want to treat this as numeric data, so type is especially important.
Example
So the correct way would be to take the value of each of the quantity field and add them together. and the resulting value you can convert to a string - which is what the textbox text property should be.
If you turn Option Strict On at the top of your source code then this will ensure that you have the correct types and will not do the implied conversions.
You can use the VB specific Conversion methods Val, Cstr etc. or you can use the .NET Ctype method to cast to a specific type. Or mix and match them as I did above.
Either way its ensuring that you using numeric types with the + operator and ensuring your converting to a string type to set the text property.
Please dont, simply use the corect data types and the correct operator for VB. Turning Option Strict On will identify issues.
If your adding numbers a simple + will be perfectly sufficent, If you concatenating string then use the & operator
All inputs and outputs in the computer MUST be strings. If a number value is needed for math processing, the string representation must be converted into a number type. After processing, the result must then be converted back into a string for display.
All textboxes accept only a string/text input. You can use the TryParse method to convert a numeric string into a number type. For example:
Integer.TryParse(TextBox1.Text, inum)
or
Double.TryParse(TextBox2.Text, dnum)
Then do your math processing with the number values (which should all be of the same type) and convert the answer to a string for display in another textbox. For example, assuming answer, num1, and num2 are all integers (or all doubles):
answer = num1 + num2
TextBox3.Text = answer.ToString()
Note, if the text input was non-numeric or blank, the TryParse method will return 0 without generating a run-time error.
I really wish they had done away with using + to concatenate strings....
Legacy issues 
Unfortunately this seems to cause problems to beginners frequently who dont seem to consider data type.
I know there are quite a few posts here already but i thought i would put my 2 cents in. It may be helpful.
If you have the variables declared already then just use the variable names
txttotalq.text = variable1 + variable2 +variable3 +variable4 + variable5 + variable6
I just noticed that you specifically said the values are already declared so why not use them.
if you want to use this many times you could combine all the variables into a single variable
dim totalvariables as integer = variable1 + variable2 +variable3 +variable4 + variable5 + variable6
Then call it when you want it
txttotalq.text = totalvariables
Anyway, take it for what it's worth, if anything it may save you a lot of code clutter
I hope it helps in some way
The answer by Js06 is unfortunately incorrect, as an answer the result does not depend upon whether a variable is declared already or not but is absolutely related to the type that the value (whether declared or not) is and the operator being used.
Here's some examples to demonstrate the problem.
With Option Strict Off (which is default)
Dim x1 As String = "10"
Code Snippet
Results in Z being 1010
Dim x1 As Integer = 10
Code Snippet
Results in Z being 20
Why, because of the types used with the + operator and an implied conversion that takes place.
In order to eliminate the problem,
1. Use Option Strict On
2. Ensure you use the correct data types
3. Ensure you use the correct operators with those types.
So in the above example if I wanted to do numeric addition
Dim x1 As Integer = 10
Code Snippet
Will Result in 20
If I wanted to do string concatenation
Dim x1 As Integer = 10
Dim y1 As Integer = 10 Dim z As String = Ctype(x1,string) & ctype(y1, string)
Will Result in "1010"
You can try the above examples using literals, constants, enums or variables and the same behaviour is evident such as
Dim z As Integer = "1" + "10"
Will Result in "110"
In the original code sample provided, they are using the + operator with the Textbox text property which returns a string value. This in conjunction with the + operator will result in string concatenation and not numeric addition as is desired. Casting the text properties to a numeric type would result in numeric addition occuring and the numeric result would need to be cast back to a string to rest the result textbox text property.
Using Option Strict will highlight this issue.
I can't quite see how my answer was any different than the example you gave.
Dim x1 As Integer = 10
Dim y1 As Integer = 10 Dim z As Integer = x1 + y1Will Result in 20
This would be the same since the values in each of the textboxes were already declared as integers. You wouldn't be using a string (text) you would be using an integer.
Your example states declared integer1 + declared integer2 = the sum of the 2 declared integers.
Isn't that what my example shows
You don't have to use the text from the boxes, you directly use the variables.
Sorry you got me going on this one when you said i was wrong. look at my example and i don't think you can say that it is any different than what you posted using integers. It bothers me when I give an example, then I'm told my example is wrong and then the same basic example is given. I can't quite figure this one out. The only reason i gave this example is because i have used it and it worked. And apparently you agree. You typed out the code that is already in decimator's app, he/she was trying to use "text" instead of the values. No disrespect to anyone, i took this one a little personal.
If anyone else can see a difference between this let me know. I think I need to take a break from posting to questions for a while.
How is the answer different.
Because in the above example I have explicitly stated the type to be a numeric type. Therefore the + operator would cause a numeric operation. However the text property of the textbox returns a string an integer. It doesnt matter if textbox contains a integer value as when you use the text property you are using a string. So if you do
DIm x as integer = Textbox1.Text + Textbox2.Text
Declaration is NOT the key concept here, it is the data type and operator. If you have specifically declared a type as a numeric type then the operator would result in numeric operator. The concept is specific to data type / operator . In the original posting the person was talking about using the Textbox Text property.
In your example
txttotalq.text = variable1 + variable2 +variable3 +variable4 + variable5 + variable6
If I was to apply this to the original posting then the types are
String = String + String + String
The values populated into the textbox may be integers but they implicitly use a tostring method to populate the text property and this property has a string value.
So your response was incorrect as it makes no reference to the specific types of the variables. If they are all string then the + operator will result in concatenation. If they are numeric types they will result in numeric addition - this was the original problem.
The difference in my example is that
declared integer1 + declared integer2 = the sum of the 2 declared integers.
which is specifically stating the types. This IS the key concept.
This was why the original examples use Val, Cint, Ctype, TryParse in order to cast the string type to a numeric type so the + operator carries out numeric addition.
So you answer is in fact wrong as its not addressing the key issue which is the data types. It does not matter if you are using a declared variable or not, its the type that matters. You can try the concept with Literals, Enums, Constants or variables and the same concept will still apply - it doesnt matter it is a declared variable.
"DATA TYPE AND OPERATOR USAGE IS KEY TO UNDERSTANDING THIS BEHAVIOUR, NOT WHETHER ITS DECLARED VARIABLE OR NOT"
Use the following example
Option Strict On
Public Class Form1
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim i As Integer = 1
Dim j As Integer = 2
Me.TextBox1.Text = i + j
End Sub
End Class
Your answer depends upon implied conversion which occurs when you dont have option strict on. Option Strict On is highlighting you data type issues showing you that your code may work because of implied conversion if you have option strict off.
Also if you are using textboxes then its likely the user is going to change some values, otherwise you may use a label. The fact that a textbox textbox returns a string, and sure I can put some code in to validate the text can be converted to a numeric value. Using the text property always returns a string, irrespective of if you populate it from integer, single, double.
This occurs with option strict off as it will use implicitly call the tostring method.
Actually I like the "+" concatentation operator because it's not as forgiving as the "&" and you have to use correct datatypes. It's a little like option Strict.