To find mn(x), we multiply the given functions m(x) and n(x), to get (x^2 + 3)(5x + 9) = 5x^3 + 9x^2 + 15x + 27. No further simplification is necessary.
This is choice A. Note that by inspection, it can be seen that m(x) * n(x) should result in a cubic expression, and only A has a cubic term 5x^3, while the rest only have up to quadratic (x^2) terms.
