(f*g)(x) means multiply the rules of each function.
(f*g)(x) = (6x + 9) × [tex] x^{4} [/tex]
= [tex]6x( x^{4} )+9( x^{4} )[/tex] ← Distributive property
= [tex]6 x^{5} +9 x^{4} [/tex] ← remember add exponents on like base when multiplying
ANSWER: [tex]6 x^{5} +9 x^{4} [/tex]