Two charges each equal toq$ are kept at x=-a and x=a on the x-axis.

Two charges each equal to $\mathrm{q}$ are kept at $\mathrm{x}=-\mathrm{a}$ and $\mathrm{x}=\mathrm{a}$ on the $\mathrm{x}$-axis. A particle of mass $\mathrm{m}$ and charge $\mathrm{q}_{0}=\frac{\mathrm{q}}{2}$ is placed at the origin. If charge $\mathrm{q}_{0}$ is given a small displacement $(\mathrm{y}<<\mathrm{a})$ along the $\mathrm{y}$-axis, the net force acting on the particle is proportional to