Calculating Based on Checkbox Value

ok I have simple situation, one checkbox and one text field. If the check box is checked, I want the value of the text field to be 3(just an example).

I'm new to formcalc
on the text field calculate, this is what I have, and ofcourse it doesn't work.

If cbquickbooks.value=1 then(txtquickbooks.rawValue=3)

Thanks!