Consider three very large, parallel planes of charge that are equally spaced as shown in the figure below. These planes are insulators, and each is charged uniformly with +188 µC on every square centimeter of area on the top surface of each plane. Use Gauss's law to determine the field at points A, B, C, and D. (Assume up is the positive direction, be sure to include the sign to indicate the direction of the field.)
E= Sigma/(2*Permittivity of Freespace)
I got the correct answer for B and C being 1.06e11 N/C and -1.06e11 N/C respectively. I can't figure out how to get A and D. I am fairly certain they will have the same magnitude with different signs.