[tex]\bf T=2\pi \sqrt{\cfrac{L}{g}}\implies \stackrel{\textit{squaring both sides}}{(T)^2=\left( 2\pi \sqrt{\cfrac{L}{g}}\ \right)^2}\implies T^2=2^2\pi^2\sqrt{\left( \cfrac{L}{g} \right)^2} \\\\\\ T^2=4\pi^2\cfrac{L}{g}\implies T^2=\cfrac{4\pi^2L}{g}\implies gT^2=4\pi^2L\implies g=\cfrac{4\pi^2L}{T^2}[/tex]
