Ok so when combining functions like these, you need to that the value inside your parentheses is the new x that you replace in the corresponding function.
For example f(g(x)) would technically be f(-2x) where you would replace all of your x values in f(x) to get f(g(x))= -2(-2x) which is equal to 4x when simplified.
Luckily, since both of your functions are the same thing, this means that g(f(x)) is also 4x.
Hope that helps!