So Kaden G.,
you do just what you always do for an algebraic expression with nested parentheses: solve it from the inside out!
So for "inside" you have g(x). That is (5x - 1). And you stick that whole expression into the "outside" expression for f(x) whereever x is: that becomes 3(5x-1) + 5. Now do that multiplication, and simplify it, and what do you get?
-- Cheers, --Mr. d.