\(f_{ }=k\ \frac{Q_3\ Q_1}{r_{31}}=\left(9x10^9\ \frac{nm^2}{c^2}\right)\left(\frac{\left(1.602x10^{-19}c\right)\left(-1.602x10^{-19}c\right)}{\left(0.53x10^{10}m\right)^2}\right)=123N\)