A solid metal ball of radius 1.6 cm is located near a hollow plastic ball of radius 2.1 cm bearing a uniformly distributed charge of -9 nC on its outer surface. The distance between the centers of the balls is 8 cm.
What is the electric field at the center of the metal ball due just to the charges on its surface? Assume the +x axis is the right, the +y axis is up and the +z axis is out.