Answer
$F = \frac{1}{4\pi\varepsilon_0} \frac{|q_1 q_2|}{r^2}$, where $\frac{1}{4\pi\varepsilon_0} \approx 8.99 \times 10^9\text{ N}\cdot\text{m}^2/\text{C}^2$.
Explanation
Like charges repel and opposite charges attract along the line connecting their centers, obeying Newton's third law ($F_{12} = -F_{21}$).