Answer
$
y=-\frac{1}{9}\cdot(x+6)^2+9
$
Work Step by Step
The vertex is the point $(h, k)=(-6,9)$, so the equation is in the form
$$
y=a(x+6)^2+9 .
$$
An $x$-intercept is -15, so $y=0$ when $x=-15$. This gives
$$
\begin{aligned}
a(-15+6)^2+9 & =0 \\
81 a & =-9 \\
a & =-\frac{1}{9}
\end{aligned}
$$ Hence $$
y=-\frac{1}{9}\cdot(x+6)^2+9
$$
