How to subtract 2 fields

Hello,

I know that this is a simple question, but I don't get to make this run. :(

I have a form with some fields, and would like to subtract two of it, as described below:

Form:
Baixas

Fields:
C_digo
Nome_Raz_o_social
Data_vencimento
Data_pagamento
Valor_bruto
Taxa
Valor_l_quido

I need that the value on Valor_l_quido equals to Valor_bruto - Taxa. Is this possible?

I also don't know if the subtracion can be displayed on the form itself or only in a view.

So, I would be very grateful if someone helps me. :)