.
Use the Coulomb law.
On Coulomb law, see this Wikipedia article
https://en.wikipedia.org/wiki/Coulomb%27s_law
When you learn the subject, the remaining calculations will be an easy, a pleasant and useful exercise to you.
When you solve such problems, knowledge of units is of great importance.
You may learn about the units from your textbook, or from the referred Wikipedia article.
Pay special attention to it and do not miss this issue (!)
Happy learning (!)