Need help on getting the resonant frequency

xuhhad

A 25.0-kg child in Red Deer pumps herself on a swing when she kicks upward on the downswing, thus changing the distance from the pivot point to her centre of gravity from 2.40 m to 2.28 m.

1)The resonant frequency of her swing before the kick is: ___ Hz, and the resonant frequency after the kick is: _____ Hz.

I don't want the answer i just want a hint or a formula i am supposed to use for this, please and thank u. Because my textbook has no examples of it.

Doc Al

Think of the child as a pendulum.

xuhhad

Yes but what do i do from there i am really confused

Doc Al

What's the period of a simple pendulum?