1. The problem statement, all variables and given/known data 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? 2. Relevant equations F = k|q||q| / (r^2) 3. 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.