Interaction of Discrete Charged Objects

[amb0027]

Homework Statement

Two charged objects Q₁ = +4Q and Q₂ = -Q are located on the x-axis. Q₁ is at the origin and Q₂ is at the x = +S m position. You also have a test charge +q_o

Homework Equations

F = kQq/d²

The Attempt at a Solution

So you find that q_o is past Q2 on the x-axis, and you set F1on3 = F2on3 and solve for d. I've done that and I'm left with the formula 3d² - 8ds + 4s² = 0. I don't know where to go from here, if I knew the value for S i could use the quadratic equation but I dont. Can anyone help me finish off this problem? Or tell me if I'm doing it wrong? Thanks

 

[ehild]

Take "s" as a known quantity and solve for d in terms of "s".

ehild