What kind of formula do I use for:

blank cell + blank cell = 0 how do I get it to not show if the value
is 0?

If the cells are A1 and B2, one way is
=IF(LEN(A1&B2)=0,"",A1+B2)

If A1=17 and B2=-17, maybe you do want zero as the result -- I would. The
above formula does that.