Answer
$f(x) =(x-3)^\frac{1}{3} +3$
Work Step by Step
$(f ∘ g)(x) = x^\frac{2}{3} + 3$
$g(x)=x^2+3$
$(f ∘ g)(x) = f(g(x)) = f(x^2+3) =x^\frac{2}{3} + 3$
Therefore lets choose $f(x) =(x-3)^\frac{1}{3} +3$
$(f ∘ g)(x) = f(g(x)) =(g(x)-3)^\frac{1}{3} +3=(x^2+3-3)^\frac{1}{3} +3= (x^2)^\frac{1}{3} +3=x^\frac{2}{3}+3$