Astronomy: Calculating Neptune's Mass from Triton's Orbit

[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.

Homework Statement

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.