Are the X and x separate variables? If they are the same variable, see the answer below.
You question is looking to solve for f(x) > g(x). This should be easy enough to plug in the equations to both sides.
f(x) > g(x)
2X > 4X + 12
-12 > 4X - 2X
-12 > 2X
-6 > X
It looks like f(x) > g(x) as long as your x is less than -6