Answer
See below
Work Step by Step
We need to show that $f(g(x))=x=g(f(x))$
$f(g(x))=f((4x)^{1/3})=\frac{1}{4}((4x)^{1/3})=\frac{1}{4}(4x)^{\frac{1}{3}\times3}=\frac{1}{4}4x=x$
$g(f(x))=g(\frac{1}{4}x^3)=(4(\frac{1}{4}x^3))^{1/3}=(x^3)^{1/3}=x^{3\times\frac{1}{3}}=x^1=x$
Thus we proved what we had to.