Self-Reproducing Rays in a symmetric Resonator

[tuanle007]

Homework Statement

Self-Reproducing Rays in a symmetric Resonator. Consider a symmetric resonator using two concave mirrors of radii R separated by a distance d=3|R|/2. After how many round trips through the resonator will a ray retrace its path?

Homework Equations

The Attempt at a Solution

i don't know where to start and equations to work with

 

[Redbelly98]

I'd start by looking up the focal length of a mirror of radius R.