A negative charge –Q is located between two positive charges, +4Q and

+16Q. The distance between +4Q and –Q is d1 and the distance between

+16Q and –Q is d2.

All charges lie along the same line and d1+d2 = 9 cm.

Calculate d1 and d2 which will provide a stable equilibrium position for

the charge –Q, i.e., the location where the total force acting on –Q due

to +4Q and +16Q is zero.

So far I have F1 + F2 = Ftotal = 0 since it specifies a stable equilibrium position. However, I can't seem to get the next step.

Any help would be much appreciated.