In the figure, the four particles form a square of edge length $a = 5.00 \, cm$ and have charges $q_1 = +10.0 \, nC$, $q_2 = -20.0 \, nC$, $q_3 = +20.0 \, nC$ and $q_4 = -10.0 \, nC$. What net electric field do the particles produce at the square's center?