I am not sure if your question is saying f(x) is x² or if it is 2x so I am giving you two answers.
If f(x) is x² the answer for f(g(x))= (x-6)²
If f(x) is 2x the answer for f(g(x))=2x-12
How did I get these answers? For fog, it basically means f(g(x)) so you just plug in what g(x) equals for the variable x in f(x)