Given 6 x -3 y =18 and 6 x + 3 y = - 12
6 x - 3 y = 18 ---------------(1)
6 x + 3 y = -12 --------------(2)
adding equation (1) and (2) you get
12 x = 6
x = 6/12 ==> x = 1/2
substitute x=1/2 in equation (1) then you will get
6(1/2) - 3 y = 18
==> 3 - 3 y = 18
==> - 3 y =18 - 3
==> - y = 15/3 ==> y = - 5
The values of x and y (1/2 , -5 ).