The force between a pair of 0.001 c charges is 200 n. what is the distance between them?

From Columb's Law which describe the electrostatic interaction.
The formula is given and looks as follows 
where:
F is force acting between charges,
q1 - charge one given in columbs,
q2 - charge two given in columbs,
r - distance between charge in meters
k - constant

First, we have to convert out formula to get r.

Now we multiplaty by  both sides of equation

Now we devide both sides by F

Next step is take square root

We have all date, we can subsitute it to our formula