Answer
$l=\frac{S-\pi r^2}{\pi r}$ where $r\neq 0$.
Work Step by Step
The given formula is
$\Rightarrow S=\pi r^2+\pi rl$
Subtract $\pi r^2$ from each side.
$\Rightarrow S-\pi r^2=\pi r^2+\pi rl-\pi r^2$
Simplify.
$\Rightarrow S-\pi r^2=\pi rl$
Divide each side by $\pi r$.
$\Rightarrow \frac{S-\pi r^2}{\pi r}=\frac{\pi rl}{\pi r}$
Simplify.
$\Rightarrow \frac{S-\pi r^2}{\pi r}=l$
Hence, the solution for the indicated variable is $l=\frac{S-\pi r^2}{\pi r}$ where $r\neq 0$.
