The figure shows five charges on a square with sides parallel to coordinate axes on the xy plot. The center of the square is located at the origin and the side length of the square is equal to a. Two negative charges of minus 2.0 nanocoulombs are placed at left corners of a square. Two positive charges of 2.0 nanocoulombs are placed at right corners of a square. A positive charge q is placed at the center of the square.

 What is the magnitude of the force on the 5.5 nC charge in the middle of the figure due to the four other charges?

-2.0 nC y 2.0 nC a + q + -2.0 nC 2.0 nC x