4x-2y = 12............(i)
+
-4x-9y = 54...........(ii)
__________ Lets eliminate x, by adding (i) and (ii).
0x -11y = 66
-11y = 66
y = 66/-11
y = -6
Let us substitute y = -6 into (i)
4x - 2y = 12
4x - 2*(-6) =12
4x + 12 = 12
4x = 12 - 12
4x = 0
x = 0/4
x = 0
Therefore x = 0, and y = -6
Hopes this helps.