Need form field to calculate sales tax

[Alex Barnes]

I am needing a 7% sales tax dollar amount to be auto calculated from the total dollar amount of box 29 (text25) and calculated into box 51(text47) 

Can somebody walk me step by step on instructions how to do this?

Can someone post an exact script to post into either simplified field notation or custom calculation script? 
Also, how do I find out what the field boxes names are? 

[Jasa Pembuat WEB]

Tanks Info

[Nesa Nurani]

As simplified field notation of tax field enter: Text25*.07

then add that field to total same as you add other fields to total.

You can see field names when you enter 'prepare form' mode, you can see names in  your photo.

[Alex Barnes]

Thank you! It worked! One more question:

I want to calculate the same 7% but I want to take both totals from Text26 and Text28 and calculate sales tax into Text31

In Text31 properties under simple field notation I tried this: Text26,Text28*.07

But it didnt work, can you advise? 

Thank-you!!! 

[Nesa Nurani]

Try like this:

(Text26+Text28)*.07