Electric Potential Difference Created by Point Charges

[BoogieL80]

I'm working on the following problem:

The drawing shows four point charges. The value of q is 2.0 µC, and the distance d is 0.74 m. Find the total potential at the location P. Assume that the potential of a point charge is zero at infinity.

http://www.boomspeed.com/boogiel80/physics19.3.gif

I converted my charge to 2.0e^-6. I used the formula V= kq / r and came up with values of -2.43e⁴ for each of the parallel charges and the same numbers, only positive, for each of the perpendicular charges. I assumed I was suppose to add all the charges and I got a total of 0 V. However, webassign is saying that is the incorrect answer. Any input would be nice.

 

[durt]

The charge to the top left is farther away than the others.

 

[150grainhunter]

Alright I'm learning the same stuff right now so I should be able to help. The thing to remember is that these are not to be added as vectors. The positive charge q that is a distance d from point p will cancel out one of the -q charges that is a distance d from point p. So you will need to find the potential due to the positive charge q that is a distance 2d away from point p and the potential due to the -q charge that is a distance d from the point p. Add these together and you should get your answer.