How would you write y = - 3x2 + 12x - 21 in vertex form?

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