Answer
$$ l= \frac{R D^2}{k} .$$
Work Step by Step
To solve the given equation for $l$, we multiply both sides by $D^2$ and then divide both sides by $k $ as follows:
$$ R =\frac{k l}{D^2}\Longrightarrow R D^2=k l\Longrightarrow \frac{R D^2}{k}=\frac{k l}{k}\\ \Longrightarrow l= \frac{R D^2}{k} .$$
