How to get query result...

I have table with next data:
Col.A | Col.B | Col.C
-10 | A | D
-2 | A | R
-3 | B | D
-1 | B | R

What I want is to get next result:
Col.B | Col.A | Desription not in result query
A | -8 | -10-(-2)
B | -2 | -3-(-1)

I am trying for several days without any progress. Can anybody halp me? Actualy I Have a problem with result Col.A.

Thanks