Question

Kate (m = 50 kg) is swinging on a tire tied by a (light) rope (L = 3 m) to a tree limb. Her twin Ashley comes along and squeezes into the tire with her. Assume that at all times the centre of mass of the person(s) riding the tire is at the end of the rope.
a) What was the period of oscillation for Kate alone?
b) What is the period of oscillation for the twins together?

I need help please! Do it step by step and then substitute the numbers into that equation