The syntax if you already knew the sheet names would be like this:
Code:
=SheetA!H2+SheetB!H2
Since you don't know the sheet names to plug in, you have to get them from C2 and C4, where the user put in the sheet names, and you have to construct a reference using the INDIRECT function, like this:
Code:
=INDIRECT(C2&"!H2")+INDIRECT(C4&"!H2")
The INDIRECT function takes a character string and interprets it as a cell reference, you see. Does this make sense to you?