( 1)
A+B-C=3
(2)
A-B-C= 1
(3)
(1)-(2) that is 2C=2.
C= 1。
( 1)+(2)
Namely: (A+B)=4
(4)
Substituting C= 1 into (3) gives: (A-B)=2.
(5)
From (4) and (5), A=3, B= 1,
So the solution of this problem is A=3, B= 1, C= 1.