I was able to come up with the answers through trial and error, and the answer for the charge +q makes sense, but I don't understand why the answer for the -q is correct. Please help.
A positive point charge +q1, a second point charge q2 that may be positive or negative, and a spot labeled P, are all on the x axis in a straight line. The distance d between the two charges is the same as the distance between q1 and P. With q2 present, the magnitude of the net electric field at P is twice what it is when q1 is present alone. Given that q1 = +2.86 µC, determine the magnitude |q2| when q2 is the following.
a) q2 is positive.
b) q2 is negative.
The correct answer for part a is that q2 = 4p.
The correct answer for b is that q2 = 12p.
I don't see why, especially in part b. Can anyone explain?