In the figure the four particles form a square of edge length $a=6\mathrm{.}60cm$ and have charges $q_1=8\mathrm{.}41nC,q_2=-20\mathrm{.}5nC,q_3=20\mathrm{.}5nC,$ and

$q_4=-8\mathrm{.}41nC.$ What is the magnitude of the net electric field produced by the particles at the square's center?

Number

Units