What Charge and Placement Ensure Equilibrium for Two Charges?

[Mercyp]

Homework Statement

Two charges –Q0 and -3Q0 are a distance l apart. These two charges are free to move but do not because there is a third charge nearby. What must be the charge and placement of the third charge for the first two to be in equilibrium?

Homework Equations

The Attempt at a Solution

 

[Fightfish]

Condition for equilibrium: no net force acting on each charge.
Apply this condition to each of the two charges to obtain a unique solution.

If you still have any problems, we'll further discuss them when you've attempted the problem.