It is found experimentally that the electric field in a certain region of Earth's atmosphere is directed vertically down. At an altitude of 350 m the field has magnitude 60.0 N/C. At an altitude of 200 m, the magnitude is 100 N/C. Find the net amount of charge contained in a cube 150 m on edge, with horizontal faces at altitudes of 200 and 350 m. Neglect the curvature of Earth.