A charge is in equilibrium when the net force on it is zero. For charges on a line, set the net Coulomb force equal to zero and solve for position. Key result: the equilibrium point between two like charges is at the midpoint; between unequal like charges q1 and q2, it's at distance d*sqrt$\frac{q1}{(sqrt(q1)}$+sqrt(q2)) from q1. Earnshaw's theorem: No stable equilibrium position exists for a charge in an electrostatic field (in free space). Any equilibrium is unstable in at least one direction. This is why charged particles cannot be trapped using only electrostatic fields. JEE may ask: "Is the equilibrium stable or unstable?" — check the second derivative of potential energy or consider displacement in a perpendicular direction.