Question

Two charges q1 = 5 µC, q2 = -29 µC, are L = 18 cm apart. A third charge is to be placed on the line between the two charges. See more below...

How far from q1 should the third charge be placed so that the net electric force on the third charge is minimized?

The hint says to come up with an equation that represents the net force on the third charge as a function of the distance x, then use appropriate calculus to find the value of x where this net force is a minimum.