Use checkbox to add sales tax

Hi all!

I am trying to build a simple interactive form, but Javascript is evidently not to be learned in a few hours. I would appreciate any help you can offer.

I can isolate the issue as follows:

There are three fields: Text1, Text2, and Checkbox

Text1 contains an amount (e.g. $100.00)

Text2 is blank by default. 

When Checkbox is checked, it causes Text2 to populate with sales tax based on Text1 ($100.00) so would read $13.00.

I imagine the plain language formula for Text2 is "If Checkbox value = true then Text2 value = (Text1 * 0.13) else ""

but I have no idea how to actually implement this. 

Thanks so much in advance!

Chris