Find the vertex of this parabola y=-x^2+6-8

Hello,

Your equation has a mistake: it is y=-x²+6x-8.

y=-x²+6x-8
 =-(x²-6x)-8
 =-(x²-6x+9-9)-8
 =-(x-3)²+9-8
 =-(x-3)²+1

Vertex is (3,1)

An other way more speedy:
y=-x²+6x-8
y'=-2x+6 =0==>x=3 and y=-3²+6*3-8=1

She will need 5/8 bags for each square yard.