You need to remember the formula constant of simple pendulum such that:
k = LT2
You need to consider the value of constant equivalent to g4π2 (g expresses the gravity acceleration)
You need to set the equations g4π2 and LT2 equal such that:
LT2 = g4π2
You need to find time period such that:
g × T2 = 4 π2 × L
T2 = 4π×L2g
⇒ T = 2π √Lg