In the figure the four particles form a square of edge length a = 5.30 cm and have charges q1 = 7.98 nC, q2 = -20.9 nC, 93 = 20.9 nC, and 94 = -7.98 nC. What is the magnitude of the net electric field produced by the particles at the square's center? 91 94 Number Units V