Let f(x)=ax+b. Then 4g(f(x))=12(ax+b)2+8(ax+b)−12.
Comparing with 3x2−32x+72:
x2: 12a2=3⇒a=±21
Constant: 12b2+8b−12=72⇒3b2+2b−21=0⇒b=37 or b=−3. Since f(0)=−3, b=−3.
x: 8a(3(−3)+1)=−32⇒−8a=−4⇒a=21.
So f(x)=2x−3.
g(2)=3(4)+4−3=13.
f(g(2))=f(13)=213−3=27.