Four charges are placed at the corners of a square. If q1 = 45.0 x 10-9 C, q2 = -28.0 x 10-9 C, q3 = 18.0 x 10-9 C, q4 = -38.0 x 10-9 C and the side of the square is a = 0.27 m. What would be the magnitude of the electrical field in the middle of the square? Answer in units of N/C.