Physics problem, help...just tell me how to do it, you don't have to tell me the answer?

Question

The sun rotates every 200 million years about the center of the milky way galaxy at a distance of 30,000 ly from the center. What is the mass of the galaxy and what assuptions were made?

Answer 1

Use
GM/R^2 = v^2/R = Rω^2 = 4π^2R/P^2
M = 4π^2R^3/(GP^2)
Substitute the numbers and apply the appropriate conversion factors
Assumptions:
1. G is universally constant
2. The sun's orbit is circular (required for this simple derivation, but not in the general case)
3. The mass of the galaxy outside the sun's orbit is negligible.