Two small insulating spheres with radius 8.50×10^-2 are separated by a large center-to-center distance of 0.450 . One sphere is negatively charged, with net charge -2.20 mu C , and the other sphere is positively charged, with net charge 3.20 mu C . The charge is uniformly distributed within the volume of each sphere. What is the magnitude E of the electric field midway between the spheres?