The electric force acting between the two charges is - 2.1 * 10^10.
Step-by-step explanation:
An electric force is exerted between any two charges. Objects with the same charge, having both positive negative charges repel each other, and objects with opposite charges will attract each other.
The formula for finding the electric force is given by,
F = (k q1 q2) / r^2
= (9 * 10^9)(4.2 * 10^-3)(-5 * 10^-3) / (3 * 10^-3)^2
F = -2.1 * 10^10.