## Calculus (3rd Edition)

$c=5/3$.
Since $f(x)$ is continuous, then it is continuous at $x=5$; thus we use the property that $$f(5)=\lim_{x\to 5}f(x)$$ where $f(5)=4(5)+2c=20+2c$ and $\lim_{x\to 5}f(x) =5^2-c$ Hence, we get $$25-c=20+2c\Longrightarrow 3c=5.$$ That is, $c=5/3$.