Electric Potential in a triangle

[Zywo]

Homework Statement

What is the electric potential at the point indicated with the dot in the figure?
The dot is in the center of an equilateral triangle with sides of 3 cm. The top point is a +2 uC charge and the bottom points are both -1 uC charges.

Homework Equations

V= kq/r

The Attempt at a Solution

I used 1.299 cm the distance from the center point to the charges. I then plugged in the values for each point and added them up. I DID however use +1 for the -1 uC charges because they add to the electric potential right?

 

[Delphi51]

You must use -1 x 10^(-6) for the -1 μC charge. The potential from the negative charge tends to cancel the potential from the positive charge. If you imagine the + and - charges slowly moving toward each other, the potential will not change suddenly when the charges contact and cancel each other out.