Three point charges lie in a straight line along the $y$ -axis. A charge of $q_{l}=-9.0 \mu \mathrm{C}$ is at $y=6.0 \mathrm{m}$ and a charge of $q_{2}=-8.0 \mu \mathrm{C}$ is at $y=-4.0 \mathrm{m} .$ The net electric force on the third point charge is zero. Where is this charge located? (See Sample Problem 17C.)