How do i put f (x) = - x^2 + 8x + 9 in vertex form?

Vertex form is y=a (x-h) ²+k

complete the square

f (x) = (-x²+8x) + 9

undistribute - 1

f (x) = - 1 (x²-8x) + 9

take 1/2 of - 8 and square it to get 16, then add neagative and positive inside

f (x) = - 1 (x²-8x+16-16) + 9

complete the square

f (x) = - 1 ((x-4) ²-16) + 9

expand

f (x) = - 1 (x-4) ²+16+9

f (x) = - 1 (x-4) ²+25

vertex form is

f (x) = - 1 (x-4) ²+25

The answer would be y = (x-4) ^2+25