How do you use Coulomb’s law?

Hint: As a first step you will have to provide a statement of Coulomb’s law. Then, from the statement, you could get the relation of the coulomb’s law. Thereby, you will be able to give the quantity calculated using Coulomb's law. Also, mention the difference between attractive and repulsive force.

Complete Step by step solution:
In the question we are asked as to how we could make use of Coulomb's law.
You may recall that Coulomb's law could be considered as one of the fundamental laws in physics.

The Coulomb's Law is the law that is used to determine the electrostatic force between the two charges. Let ${{q}_{1}}$ and ${{q}_{2}}$ be two charges that are kept at a distance r, then electrostatic force between them is found to be directly proportional to the product of the two charges and inversely proportional to the square of the distance between them. This is the statement of Coulomb’s law and it can be given by the following relation,
$F\propto \dfrac{{{q}_{1}}{{q}_{2}}}{{{r}^{2}}}$
$\therefore F=k\dfrac{{{q}_{1}}{{q}_{2}}}{{{r}^{2}}}$
Where, k is the proportionality constant known to have a value,
$k=\dfrac{1}{4\pi {{\varepsilon }_{0}}}$

Thus we could find the force between any two charges separated by r distance using coulomb’s law. You could also find the distance between the two charges if the value of force between them is known.

Note:
We know that the electrostatic force can be attractive and repulsive. The force will be negative when the given charges have opposite signs and it will be positive when both the charges are of the same sign. So, we could conclude the force is attractive if the force is negative and repulsive otherwise.