A charge of 3.75 10-9 C is located 3.1 m from a charge of -3.10 10-9 C. Find the magnitude of the electrostatic force exerted by one charge on the other.
Coulumb's Law is the equation to use.
F = k Q1 Q2/R^2
The Q's are the two charges.
R is the separation.
k is the Coulomb constant, which you should look up.
Opposite-sign charges attract