...of her motion? A 34.0- kg child swings on a playground swing 3.5 m long. What is the period of her motion?

part 2 If her older brother, who weighs twice as much as she does, rides in the swing instead, what is the period of the motion? Assume the center of mass to be at the position of the seat in both cases.

i have done a problem like this before and i have tried setting it up and i cant seem to get it