Pendulum problem?

Question

A visitor to a lighthouse wishes to determine the height of the tower. She ties a spool of thread to a small rock to make a simple pendulum, which she hangs down the center of a spiral staircase of the tower.The period of oscillation is 9.40s. What is the height of the tower?

Answer 1

you should use the formula T=2pi*square root of( l/g )
L = length of the cord (in this case height of the lighthouse)
T = "period of oscillation"
g= gravitational force (10m/s^2)
pi=pi=about 3.14

Answer 2

Taking g as 9.80 m/s/s

This gives-:
9.40 x 9.40 x 9.80
----------------------- = 21.934 metres
4 pi x pi