Three point charges are placed on the $x$ -axis. A charge of $3.00 \mu \mathrm{C}$ is at the origin. A charge of $-5.00 \mu \mathrm{C}$ is at $20.0 \mathrm{cm},$ and a charge of $8.00 \mu \mathrm{C}$ is at $35.0 \mathrm{cm} .$ What is the force on the charge at the origin?