Only thing you have to do is focus on what is asked to do. Note that each row and each column gives rise to a linear equation on three or four variable. But we need (A - B) only, so we have to manipulate only those rows or columns which leads us to this particular relation.
Take the first row,
- (A + A + B + C) = 11 => (2A + B + C) = 11 ................... (1)
- (A + B + C + B) = 5 => (A + 2B + C) = 5 ....................... (2)
- (2A + B + C) - (A + 2B + C) = (11 - 5)
=> (A - B) = 6
