Three point charges are arranged along the x axis: q1 = −4.43 nC at x1 = 199 mm, q2 = 2.33 nC at x2 = -302 mm, and a positive charge q3 at the origin. What is the value of q3 if the net force on this point charge has magnitude 4.50 µL?

Answer 1

I have never heard of force measured in µL. Is that supposed to be microliters? Check again.

Then write the equation

F = k q3 (q1/x1^2 - q2/x2^2), making sure you use the negavive value for q1. The forces due to q1 and q2 will be in the same direction. k is the Columb constant. Solve for q1
Make sure the q's are in Coulombs, the distances x are in meters, and the force in Newtons.