Homework Statement

Here is the problem :
http://i1200.photobucket.com/albums/bb327/loba333/untitled.jpg
ill write it out again aswell
A particle with charge +9 μC is placed at the origin, and another particle of charge –2 μC is at x = L. At what two points is the net electrostatic potential equal to zero?

V=KQ/r

The Attempt at a Solution

I could understand if they were the same sign charges then there would be an area of 0 field strength.
Could someone please explain to me whats going on and how to solve it.

Cheers

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