Create the formula and use:
{@a} + {@b}
If you get an error, try posting technical information, note that your post is devoid of anything about the software used, dataqbase, what's in the formulas, etc.
If @A or @B use summary functions, then you'll want to use a different method to summarize them, such as the exact same syntax within, or variables within a summing formula, and then displaying them later.
-k