Ah cheers got it now, misread the question was thinking M was the total mass of sphere rather than just the stars contributions.. M(r)_{dm} = 4πρ_0r^2_0r
The Attempt at a Solution
I think I may of been too general with the volume in finding M_0
I'm assuming r_0 = r_M although I'm not sure why different notation is used thougj
http://imageshack.us/photo/my-images/3/gravscan.jpg/