How do I put this typ of equation into an excel spreadsheet.

A6(variable)+____ =65

(A6+B6=$C$6)

I need to set up a spread sheet so when I put a number in the A6 area the B6
area is always populated with the right number. 65(C6) is a constant. I hope
I explained this correctly I know this may be simple for some.... Thanks for
any help.