Two charged particles are located on the x axis. The first is a charge +Q at x = -a. The second is an unknown charge located at x = +3a. The net electric field these charges produce at the origin has a magnitude of 2keQ / a2. What are the two possible values of the unknown charge?

I honestly have absolutely no idea where to begin solving this problem. I know the equation for the electric field (kQ/r2), but I have no idea how to work with the information given to get a net electric force. Can anyone show me where to start?