Four charges of equal magnitude q are placed at the corner of a square with sides of length 2r as shown in the figure. Two charges are positive +q, while the other two are negative -q. We are interested in a point P at the center of the square.

Write an expression for the electric potential at the point P, in terms of the magnitude of charge q, the length r, and any relevant physical constants. Please show all your work.

+ 9 -q 2r X 2r 2r P + 9 + -q 2r