Question

T = 2π √l/g Re-arrange to make l the subject.

Answer 1

$T = 2 \pi \sqrt{ \frac{l}{g}}$
$\frac{T}{2 \pi} = \sqrt{ \frac{l}{g}}$
$( \frac{T}{2 \pi} )^{2} = \frac{l}{g}$
$l = g \times ( \frac{T}{2 \pi} )^{2}$