Zhb,
Set up a variable for each textbox item, then simply add them to the formula. For example:
Dim tb1 as Double
Dim tb2 as Double
Dim tb3 as Double
tb1 = cdbl(me.textbox1.value)
tb2 = cdbl(me.textbox2.value)
tb3 = cdbl(me.textbox3.value)
textbox6.value = (tb1 + tb2) * tb3 - (tb3 *100)
Obviously, my example is a made up formula and I didn't create variables for ALL of your textboxes, but I think you can figure it out from there.
As far as WHERE to add it, this really is up to you and based on need. For example, it may not even be in the TextBox6 code. Instead, it might be in the TextBox5_Exit (meaning as soon as you exit TextBox5, the code runs. It's hard to answer this part of your question without seeing your userform, and explaining what you need it to do.
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