# What is the vertex of y=-x^2 - 18x + 9?

Jul 6, 2017

$y = - {x}^{2} - 18 x + 9$

Factor out the coefficient of the highest power of $x$ ($a$ value):
$y = - \left[{x}^{2} + 18 x - 9\right]$

Rewrite what's inside the brackets using vertex form
$y = - \left[{\left(x + 9\right)}^{2} - 81 + 9\right]$

$y = - \left[{\left(x + 9\right)}^{2} - 72\right]$

Finally distribute the negative sign throughout the brackets
$y = - {\left(x + 9\right)}^{2} + 72$

$\textcolor{b l u e}{\text{The vertex of the parabola is at } \left(- 9 , 72\right)}$