In the figure the four particles form a square of edge length a = 7.10 cm and have charges $q_1 = 7.26 \, nC$, $q_2 = -18.9 \, nC$, $q_3 = 18.9 \, nC$, and $q_4 = -7.26 \, nC$. What is the magnitude of the net electric field produced by the particles at the square's center?