f(g(x)) >> In f(x), replace all of the x's with g(x)
f(g(x)) = 6g(x) -11
f(g(x)) = 6(4x^2) -11
f(g(x)) = 24x^2 -11
There's nothing x can't be- no fraction bars (dividing by 0) or square roots- so x can be anything. The domain (possible x values) should be all real numbers.