These used to confuse me greatly!
Remember that f(g(x)) means that we are placing g(x) in place of x in f(x), much like f(3) would mean replacing x with 3, right?
h(x) = (4x - 3)3
Well, if we started with f(x) = x3 and replaced x with 4x - 3, then we'd end up with h(x).
f(x) = x3
g(x) = 4x - 3
f(g(x)) = (4x - 3)3
Hope this helps!