g (n)=n^3-3n^2 h (n)=n-1

Question

find (g • f)(a)

Answer 1

If you mean f(n)=n-1 and if (g·f) denotes multiplication (not composition) then you simply multiply the polynomials  

 

(g·f)(a) = g(a) · f(a) = (a^3 - 3a^2) · (a - 1) = a^4 - 3a^3 - a^3 + 3a^2 = a^4 - 4a^3 + 3a^2