ElectroStatic Equilibrium problem

[RandellK02]

Homework Statement

particle 1 of charge q1 (at origin) and particle 2 of charge q2 = 3q1, are held at separation L on an x axis. If particle 3 of unknown charge q3 is to be located such that the net electrostatic force on it from particles 1 and 2 is zero, what must be the (a) x and (b) y coordinates of particle 3?

Homework Equations

F=k|q1||q2|/d²

The Attempt at a Solution

I assumed q1 and q2 are like charges so I put q3 in the middle. This could be my mistake since I am getting incorrect answers for x, I do know y = 0.
If q3 does belong between the two charges on the X axis, then I am not setting up my d² part of Coulombs law. Could someone point me to the right direction?
Net Force = 0
F₃₁=k|q3||q1|/?²
F₃₂=k|q3||q2|/?²

 

[wukunlin]

if the particle is at distance d away from q1, what distance is it between the particle and q2? assuming this is a one dimensional problem

 

[RandellK02]

if the particle is at distance d away from q1, what distance is it between the particle and q2? assuming this is a one dimensional problem

Okay say, if distance between q3 and q2 =d than distance between q3 and q1 is L - d correct? So...
kq₃q₁/(L-d)²=kq₃q₂/d²
=>q₁/(L-d)²=q₂/d²

If the above steps are correct... is it safe to assume to use the fact: q₂=3q₁ to eliminate the two charges?

 

[wukunlin]

yes :)