First tip: in gravitational motion, the force on an object is not important; the object's acceleration is. As the acceleration of an object in a gravitational field is independent of its mass, any calculations of Triton's orbital radius or speed or whatever must be independent of its mass.
Second tip: don't rote learn formulas without knowing how to use them xP
Third tip: you've actually answered question a by yourself in the question
Here's an example of what I mean.
Let's say we want to derive the special case of Kepler's third law for circular orbits. Then, as the gravitational force provides the centripetal acceleration, we have mv^2/r = GmM/r^2
Or, if you want, v^2=GM/r
Note how the m that cancelled was the mass of the satellite and not Neptune.
Then, v=2pi*r/T, so v^2=4pi^2*r^2/T^2=GM/r
r^3/T^2 = GM/4pi^2
Do the rest yourself now