The data in a text box is a string; hence you cannot do calculations with it directly. Number calculations such as addition/multiplication etc can only be carried out on data types such as integer, single, long etc.
To do what i think you're trying to do in your vb code:
You have to create a variable with data type integer. Then say variable = textbox.text
You can then do calculations on the variable, eg variable = variable * 100
If you want the result in the text box to show again, obviously you'd have to make the text box display the variable as its text