Question

Write y=-3x^2+12x-21 in vertex form

Answer 1

The vertex form of any function is : f(x) = a(x-h)^2 + k
 Hence the correct option would be: A): "-3(x-2)^2-9"

Answer 2

Complete the square: 
y = 3x^2 - 12x + 4 
y = 3(x^2 - 4x) + 4 
y = 3(x^2 - 4x + 4 - 4) + 4 
y = 3(x^2 - 4x + 4) - 12 + 4 
y = 3(x - 2)^2 - 8