The force exerted by Q2 on Q1 can be found using Coulomb's Law:
$F_{21} = k \frac{Q_1 Q_2}{r_{21}^2}$
where $k = 8.99 \times 10^9 Nm^2/C^2$ is the electrostatic constant, $Q_1 = 10.0 \times 10^{-6} C$, $Q_2 = -10.0 \times 10^{-6} C$, and $r_{21} = a = 0.20
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