Physics  mass of a galaxy
posted by Farah .
A star at the edge of the Andromeda galaxy appears to be orbiting the center of that galaxy at the speed of about 200km/s. The star is about g*10^9AU from the center of the galaxy. Calculate a rough estimate of the mass of the Andromeda galaxy. Earth orbital radius is 1.40*10^8

1 a.u. = 1.40*10^8 km = 1.40*10^11 m
For the mass of the entire galaxy, M, you will have to assume a spherical distribution of mass, with the star in question at the outside, at distance R.
In that case, the centripetal acceleration of the star is
V^2/R = G M/R2
Solve for M.
M = R V^2/G
You did not type the distance R correctly. Is the "g" supposed to be a 9?
G is Newton's universal constant of gravity, 6.67*10^11 N*m^2/kg^2