Answer
$$L=\frac{RA}{\rho } .$$
Work Step by Step
To solve the given equation for $L$, we multiply both sides by $A$ and then divide both sides by $\rho$ as follows:
$$ R=\frac{\rho L}{A}\Longrightarrow R A=\rho L\Longrightarrow \frac{RA}{\rho }=\frac{\rho L}{\rho }\\ \Longrightarrow L=\frac{RA}{\rho } .$$
