The vertex form of that expression would be y = 3 ( x +2 )squared - 25. This is done by transporting first the -21 to the other side then simplifying the 3(x2 + 4x). Then making the "x2 + 4x" a perfect binomial we need to add 4 and that makes it "x2 + 4x + 4". Then after that we now have the vertex form of y = 3 ( x +2 )squared - 25