In the figure the four particles form a square of edge length $a=6\mathrm{.}20cm$ and have charges $q_1=7\mathrm{.}89nC,q_2=-18\mathrm{.}7nC,q_3=18\mathrm{.}7nC,$ and

$q_4=-7\mathrm{.}89nC.$ What is the magnitude of the net electric field produced by the particles at the square's center?