Respuesta
$F = k_e \frac{|q_1 q_2|}{r^2}$, where $k_e = \frac{1}{4\pi\varepsilon_0} \approx 8.99 \times 10^9 \text{ N}\cdot\text{m}^2/\text{C}^2$.
Explicación
The force acts along the line joining the charges; like charges repel, whereas opposite charges attract.