What formula gives the strength of an electric field, E, at a distance from a known source charge?

Electric field is a vector: it has both magnitude and direction. The magnitude gives the strength of the field.

The source charge is the electric charge that creates the electric field. The quantity of charge is denoted by the symbol q. When placed within the electric field this charge will experience an electric force. This force is denoted by the symbol F. The magnitude of the electric field is the force per charge. The strength of an electric field is denoted by the symbol E. According to the previous, the following formula can be written:

E=F/q