This looks a simple harmonic motion equation for a pendulum but the left-hand side (period of the motion) normally expressed as T has been replaced by a square root sign with no argument. g is acceleration due to gravity (about 9.8 m/s^2) and l the pendulum length. So to solve for l we need a figure on the left. Call it sqrt(x) then x=4(pi)^2l/g and l=gx/(4(pi)^2).