Answer
$13$
Work Step by Step
Expressing the equation $
f(x)=-x^2-6x+4
$ in standard form results to
\begin{array}{l}
f(x)=-(x^2+6x)+4
\\\\
f(x)=-(x^2+6x+9)+4+9
\\\\
f(x)=-(x+3)^2+13
\end{array}
With the vertex at $(-3,13)$, then the maximum value is $
13
$, which occurs when $x=-3$.
