Question

Michael stands upright and allows his right arm to swing back and forth, naturally and without forcing, like a pendulum. He measures the time for 29 oscillations (complete cycles) to be 38.6 s. Assume his arm (length L) behaves as a simple pendulum, with an effective length of 2/3 L. Find the length of his arm, L.

I did this and the answer is wrong:

frequency = 1/2pi(sq. root (g/L))
** rearranged the formula to solve for L and then plugged in g = 9.8m/s^2 and frequency = 38.6/29 = 1.2689655 Hz.