In the figure the four particles form a square of edge length $a=6\mathrm{.}50cm$ and have charges $q_1=9\mathrm{.}16nC,q_2=-21\mathrm{.}0nC,q_3=21\mathrm{.}0nC,$ and $q_4=-9\mathrm{.}16nC.$ What is the magnitude of the net electric field produced by the particles at the square's center?

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