Volgens mij heb je dat goed begrepen.
$$\frac{T}{2}=\pi \sqrt{\frac{l}{g}}$$
$$\frac{T}{2 \pi}=\sqrt{\frac{l}{g}}$$
$$\left(\frac{T}{2 \pi}\right)^2=\frac{l}{g}$$
$$\frac{T^2}{4 \pi^2}=\frac{l}{g}$$
$$\frac {4 \pi^2}{T^2} =\frac{g}{l}$$
$$\frac {4 \pi^2\cdot l}{T^2} =g$$
Groet, Jan