Four point charges are located at the corners of a square with sides of length a. Two of the charges are +q, and two are -q.
Find the magnitude of the net electric force exerted on a charge +Q, located at the center of the square, for the following arrangement of charge: the two positive charges are on the top corners, and the two negative charges are on the bottom corners.
Express your answer in terms of the variables q, Q, a, and appropriate constants. An expression not an equation.