Mass places o that it is in equilibrium

[mrknowknow]

Homework Statement

A mass,M, is fixed at a position of 3m. Another mass, 2M, is fixed at the position -1m. What position(s) can a third mass, 3M, be placed so that it is in equilibrium.

Homework Equations

The only thing I could think of is M(3)+2M(-1)+3Mx=0

The Attempt at a Solution

I tried the center of mass equation and got the same wrong answer.

 

[xophergrunge]

So that what is in equilibrium? I believe you are missing part of the question.