Two charges are on the x axis. A charge of -3microCoulombs is 3cm to the right of the origin and a charge Q is 3cm to the left of the origin. The point P is 3cm directly above the -3MicroCoulomb charge, that is, its coordinates are (3cm, 3cm). Find the value and sign of Q so that the total electric field at the point P is parallel to the x axis.

(When solving the problem I applied the equation E = kQ/r2, for electric field. However the solution key uses the equation kQ/r, (without the r being squared), which I understand is the equation for electric potential. Why do we use the equation for electric potential here when they’re asking about electric fields?)