Making a formula for figuring out speed?

A hockey puck of m m is attached to a string that pes through a hole in the center of a table. The hockey puck moves in a circle of radius r. Tied to the other end of the string, and hanging vertically beneath the table, is a m M. uming the tabletop is perfectly smooth, what speed must the hockey puck have if the m M is to remain at rest? (Use g for acceleration due to gravity, and m, r, and M as necessary.)