x - 3y = 12.....x = 3y + 12
so sub in 3y + 12 in for x in the other equation
2x + 5y = -20
2(3y + 12) + 5y = -20
6y + 24 + 5y = -20
6y + 5y = -20 - 24
11y = - 44
y = -44/11
y = -4
x - 3y = 12
x - 3(-4) = 12
x + 12 = 12
x = 12 - 12
x = 0
solution is (0,-4)
