Arranging that
-2x + y = 6
3x - y = 6
You can use the elimination method by adding those two equations
-2x + y = 6
+ 3x - y = 6
_____________
x = 12
You substitute x for either two of equations. Let's the first equation.
y = 2(12) + 6 = 24 + 6
y = 30