Answer
$c=21$.
Work Step by Step
Since $f(x)$ is continuous, then it is continuous at $x=3$; thus,we use the property that $$f(3)=\lim_{x\to 3}f(x)$$ where $f(3)=-4(3)+c=-12+c$ and $\lim_{x\to 3}f(x)=2(3)+3=9$
Hence, we get $$9=-12+c\Longrightarrow c=21.$$
That is, $c=21$.
