Answer
$l=\frac{gT^2}{4\pi^2}$
Work Step by Step
First, divide both sides of the equation by $2\pi$ to get $$\frac{T}{2\pi}=\sqrt{\frac{l}{g}}$$ Next, square both sides to get $$\left(\frac{T}{2\pi}\right)^2=\frac{T^2}{4\pi^2}=\frac{l}{g}$$ Finally, multiply both sides by $g$ to get $$l=\frac{gT^2}{4\pi^2}$$
