-3x + 3y = -18
-3x - 3y = 6 (notice how you have a -3 and +3, these cancel out) add both equations together you will end up with
-6x + 0y = -12
-6x = -12 now divide each side by -6
x= 2 (take this and plug it into either one and solve for y imma plug it into the first one)
-3(2) + 3y = -18
-6 + 3y = -18 add 6 to both sides
3y = -12 divide each side by 3
y = -4 x = 2
check: -3(2) + 3(-4) = -18
-6 + -12 = -18
-18=-18 checks out