How do you use Coulomb's law?
Coulomb's law is the formula for electric force.
The two q's shown are charges both in the standard units coulombs. The r is the distance between the two charges in meters. The K is the electrical constant,
The law of attraction, stating that opposite charges attract and identical charges repel, is what you must use to decide the direction of the force. Two positives (or negatives) will repel each other while two opposite charges (positive and negative) will attract towards each other.