 Problem Statement

The Sun rotates about the center of the Milky Way Galaxy at a distance of about 30000 lightyears from the center
(1ly=9.5×10^15m).
Part A
If it takes about 200 million years to make one rotation, estimate the mass of our Galaxy. Assume that the mass distribution of our Galaxy is concentrated mostly in a central uniform sphere.
Part B
If all the stars had about the mass of our Sun (2×10^30kg), how many stars would there be in our Galaxy?
 Relevant Equations
 ((4pi^2)r^3)/G*T^2)
A previous thread outlined the problem with a correct answer, however I don't understand where they got the formulas from. Here are the steps I've taken so far:
1. Convert 2*10^8 years to seconds = 6.3072*10^15 seconds (period,T)
2. The previous thread then went on to say you plug period and radius (which the thread said was 2.85*10^20, how is this found?) into the "formula":
((4pi^2)r^3)/G*T^2) How do you arrive at this formula?
1. Convert 2*10^8 years to seconds = 6.3072*10^15 seconds (period,T)
2. The previous thread then went on to say you plug period and radius (which the thread said was 2.85*10^20, how is this found?) into the "formula":
((4pi^2)r^3)/G*T^2) How do you arrive at this formula?