First, we want to subtract function f minus function g to get a new function:
(f-g)(x)= x+12 - (-2x)
(f-g)(x)= x+12+2x
(f-g)(x)= 3x+12
Now, we want to plug in 144 for x:
(f-g)(144)=(3)(144)+12
(f-g)(144)=432 +12
(f-g)(144)=444
Therefore, the answer is 444