Walker, Halliday & Resnick — Coulomb's Law: Problem 6

6 Three particles are fixed on an x axis. Particle 1 of charge \( q_1 \) is at \( x = -a \), and particle 2 of charge \( q_2 \) is at \( x = +a \). If their net electrostatic force on particle 3 of charge \( +Q \) is to be zero, what must be the ratio \( q_1 / q_2 \) when particle 3 is at (a) \( x = +0.750a \) and (b) \( x = +1.50a \)?