Mass of cluster question in astronomy

[jaficara]

Homework Statement

You observe that in a rich cluster of radius 2 Mpc, galaxies move with a typical speed of 1500km s^(-1). Estimate the mass of the cluster.

Homework Equations

I can't seem to find any type of formula that makes sense to me in regards to this question.

The Attempt at a Solution

I don't even know where to begin. I though of using a simple mass formula, but I can't find one that uses speed and radius as givens.

 

[chemisttree]

http://www.astro.umass.edu/~arny/jclus_agnquiz.html