Mass of cluster question in astronomy

1. The problem statement, all variables and given/known data

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.

2. Relevant equations

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

3. 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.

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