Xenosmilus

First, we can simplify f(x): $f(x)=-3x^2+6x+27$. Then, we factor out -3 for $f(x)=-3(x^2-2x-9)=-3(x-1)^2+30$.

 From this equation, we get that the vertex of f(x) is (x, y) = (1, 30).