All you have to do to solve this question is to plug in gx as any other number you would plug in to the f(x) to solve the equation.
So it would be
F(3x-8) = 3(3x-8)^2 - 5(3x-8) - 9
Then FOIL the terms in the binomial or the first term, then after expanding the first term, distribute the numbers into each term inside the bracket and then at the end gather like terms to simplify.