Finding moon B's orbital period from moon A using Kepler's third law

[Devin Longo]

Homework Statement

Two moons orbit a planet in nearly circular orbits. Moon A has orbital radius r, and moon B has orbital radius 5r. Moon A takes 50 days to complete one orbit. How long does it take moon B to complete and orbit?

Homework Equations

Kepler's Third Law

The Attempt at a Solution

I know we're supposed to attempt it, but I'm not even sure I'm heading in the right direction! Can anybody give me some insight?

 

[mgb_phys]

have you tried looking up kepler's 3rd law /