I need a formula that will subtract positive numbers from negative numbers
"and" negative numbers from positive, with the answer in the correct form,
ie; positive or negative.

All numbers refer to either a gain or loss in quantity
Row 1 values are taken from one source
Row 2 values are taken from another source
I need a formula for row 3 to give me the total gain or loss represented in
each column.

Example
A B
1 (20.5) 17 Row 1
2 1.5 (2) Minus Row 2
3 (19) 15 Equal Row 3