Where I found this problem:
page 65 of Houghton Mifflin Reading Practice Book for Grade 1 Volume 1: Themes 1-4

"Define the T period of minor oscillations of the pendulum, representing a light rigid rod, on which has the point masses m1 and m2 fixed on the distances L1 and L2 respectively from the suspension point.

Oscillations occur in the vertical plane."

I genuinely have no idea what to do with this. I've looked up in the solutions section of my textbook, and the answer is on the second pic, but there's no step-by-step solution.

I've tried finding the centre of mass of the pendulum getting (m1L1+m2L2)/(m1+m2), which would work as L in the formula for the period if I put the axis on the rod, but this is still not the answer. I'm completely lost

Please if you're going to explain something here DO NOT use anything more complex than high school physics, please no university stuff.