Equations:
4x - 2y = 12
-4x - 9y = 54
Multiple them in both sides
4x - 2y - 4x - 9y = 12 + 54
-2y - 9y = 66
-11y = 66
y= -6
If y= -6 then put y in first or second equation.
I put it in first equation
4x - 2.(-6) = 12
4x + 12 = 12
x= 0
Therefore
(x,y) = (0, -6)