Astronomy - Orbit length and Oribital Period of the moon to Mass of the Larger planet

Branches

This question has been stumping me on my Astronomy homework for the past couple of days. I've tried to input the question into any of the other formulae that we've studied, but I keep drawing a blank on it.

1. The problem statement, all variables and given/known data

Given the size of Triton's orbit(a = 355,000,000 meters) and its orbital period (P = 5.877 days), calculate the mass of Neptune.

mgb_phys

Look up kepler's third law, it does include the mass of the moon but as long as this is much less than the mass of the planet it can be ignored.

Branches

Thanks.