I can give you a hint. So when we do the composition of functions, if we plug 2 functions into each other and get x by itself, that means the functions are inversely related.
For this problem, simply find the inverse of f(x).
If you are stuck, I could provide you with a solution below...
f(x) = 8x³ - 2
y = 8x³ - 2
y + 2 = 8x³
(y + 2)/8 = x³
cube root((y + 2)/8) = x
g(x) = cube root((x + 2)/8)