taking the sum or difference of two columns and putting it in anot

I have a database that I enter a number in two columns and need the 
difference to be calculated and inserted in another. I want this to happin
in 
the form I use to enter and not in a query. I tried using a query but I
get a 
circlular reference error. I can get the right result in a query but I
can't 
get that result back to the table that I reference in my form. I believe I

need to use a "on update". please help.