Question

Given:

Two charges q1 = 4 µC, q2 = -40 µC, are L = 13 cm apart. A thirdcharge is to be placed on the line between the two charges. How farfrom q1 should the third charge be placed so that the net electricforce on the third charge is minimized?

Note- the correct answer involves finding where the rate of change(derivative) of the first equation equals that of the second. Ihave been unsuccessful in setting this up.

Any help appreciated!