How Is Enclosed Mass Derived in the Hernquist Model?

[TheSpaceGuy]

Homework Statement

A popular model for the mass distribution in a spheroid (such as a bulge) is the
Hernquist model, with density

ρ(r) = M(total)*a / 2πr(a+r)^3


M(total) is the total mass. a is constant with dimensions of length. Derive the enclosed mass
M(r)

The Attempt at a Solution

I am completely unsure on how to go about doing this? Any help is appreciated!

 

[dextercioby]

How are generally mass density and mass related ? (Hint: integral).