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1. The problem statement, all variables and given/known data
Two point charges (q1=+20 nC and q2=-40 nC) are placed 10.0 cm apart onlong the x-axis. Where can a third charge (q3=+20 nC) be placed so that the net electrostatic force on that charge is zero?


2. Relevant equations

F=k(q1q2)/r^2

3. The attempt at a solution

It's definitely not a hard problem, I'm guessing I'm supposed to set the entire equation into equilibrium. Other than that basic concept, I really don't understand how to solve for this problem. My professor only went over calculating forces between two charges.

Thanks for any help, I could really use it. :(