Finding Electric Potential at Point Marked by Dot

[Netsurfer733]

Homework Statement

What is the electric potential at the point indicated with the dot in the figure?

http://img842.imageshack.us/i/29ex26.jpg/

Homework Equations

E = V_c / d ?

V = E*s?

V = k * q / r ?

The Attempt at a Solution

I'm really rather confused by this problem. It requires vectors and angles, etc., I'm sure - but how does one factor in electric potential for this, for some arbitrary point (that has no charge, I presume?)? Can anyone guide me a bit?

 

[ehild]

The potential is defined at a point, and it is the potential energy of a unit positive point charge placed there. The potential is scalar quantity, no need to use vectors and angles. The potential at distance r from a point charge q is k*q/r. If there are more point charges, their potentials add up.

ehild