In Problem 86, the +2.0 C charge is at x = 0 and the 4.0 C charge

In Problem 86, the +2.0 μC charge is at x = 0 and the −4.0 μC charge is at x = d. Find the x-coordinates of the point(s) where the electric field is zero.

Data From Problem 86

A parallel plate capacitor has a charge of 0.020 μC on each plate with a potential difference of 240 V. The parallel plates are separated by 0.40 mm of air. What energy is stored in this capacitor?