Magnitude of a Point Charge on a Line

[theintarnets]

Homework Statement

The drawing shows a positive point charge +q1, a second point charge q2 that may be positive or negative, and a spot labeled P, all on the same straight line.
The distance d between the two charges is the same as the distance between q1 and the spot 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 = +1.19 µC, determine the magnitude |q2| when q2 is positive. Determine the magnitude |q2| when q2 is negative.

Homework Equations

Actually my problem is I don't know which equation I'm supposed to use. All I know is that E = kq/r²

The Attempt at a Solution

I tried making an equation, given that the net field at P is twice what q1 is alone. So I would then have something like:
2(kq/r²) = kqQ/2r²
I'm not sure if that's right though. I would just get 4 as an answer. And then I have no idea what to do about -q2...

 

[TSny]

The basic concept in this problem is the principle of superposition. The net electric field vector at point P when both charges are present is the sum of the electric field vectors from each charge.

$\vec{E}_{net} = \vec{E}_1 + \vec{E}_2$.

You will need to think about the directions of the vectors and how to express the distance $r$ for each charge in terms of the distance $d$ given in the figure.