Ah, function composition.

if g(x) = 4√x, f(g(x)) means that we'll have to change all instances of x in f(x) with 4√x, simple as that. Do the same with g(f(x))

A. f(g(x)) = 5(4√x)^4 - 6(4√x) + 10

B. 4√(5x^4-6x+10)