write y=-3x^2+12x-21 in vertex form
A)y=-3(x-2)^2-9
B)y=-3(x+2)^2-9
C)y=-3(x+2)^2+9
D)y=-3(x-2)^2+9
A)y=-3(x-2)^2-9
B)y=-3(x+2)^2-9
C)y=-3(x+2)^2+9
D)y=-3(x-2)^2+9