Electric Field and Potential on a Line with Opposite Charges

[silver_gry]

Homework Statement

A positive charge of +q is located 3.00m to the left if a negative charge -q2. The charges have different magnitudes. On the line through the charges, the net electric field is zero at a spot 1.00m to the right of the negative charge. On this line there are also two spots where the potential is zero. Locate these 2 spots relative to the negative charge.

Homework Equations

I don't know which equation I should use to start with. I really need help with this question.

The Attempt at a Solution

 

[Delphi51]

At the point where E = 0, the E field due to the positive charge is equal (and opposite) to the E field due to the negative charge. Write this, replace each E with the formula for E, put in the numbers for the distance. You will end up with Q1 = a number times Q2.

Now pick a point x on the line through the charges. Say, x meters to the right of Q1. Write the expression for the potential there - the sum of the potentials due to each charge. Use the multiple of Q1 in place of Q2 so the Q1 cancels out in this equation. You should be able to solve it and find x.