We have that
x - 3y = 12 and -x + y = 4
We add the 2 equations together
x - 3y + (-x + y) = 16
-> -2y = 16
-> y = -8 (1)
We plug y = -8 into -x + y =4
-> -x - 8 = 4
-> -x = 12
-> x = - 12 (2)
From (1) and (2) we could conclude that the answer is B
