Where should a negative charge be placed between two positive charges for upward force?

[Sylvia]

Homework Statement

[/B]
In the above figure, if Q1 and Q2 are positive, where could a negative charge be placed and have a vertically upward net force on it?

Homework Equations

F = k|q||q| / (r^2)

The Attempt at a Solution

I wrote the force equations for the negative charge and both positive charges, but I have no idea how to proceed from there.

 

[Sylvia]

Please do not reply, I have figured this out