A solid metal ball of radius 1.5 cm bearing a charge of -15 nC is located near a hollow plastic ball of radius 1.9 cm bearing a uniformly distributed charge of -7 nC (see the figure) on its outer surface. The distance between the centers of the balls is 9 cm. Assume the +x axis runs to the right, the +y axis runs up and the +z axis is out. What is the electric field at the center of the metal ball due only to the charges on the surface of the metal ball?