Abstract Algebra: z3[X]/f(x)

[ashley20]

Let f(x) be an irreducible, monic, quadratic polynomial in Z3[X] and F= (Z3[X]/f(x)). For any a, b in F prove that (a+b)^3 = a^3 + b^3. I know what f(x) is, there are three: x^2+1, x^2+x-1, x^2-x-1. From here, I am completely lost. Please help!