can someone explain how to get the answer? (6.21x10^9)

Question 1 0 I 2 points The scenario below shows a square with point charges at each corner. Points A, B and C each have a charge with a magnitude of +[8.51x10"'0) C. Point D has a charge of -5.63 C. Point X is at the center of the square, and the sides of the square are 6.40 m long. The magnitude of the electric field at point X is a.bc x 10Cl NIC. . input your answer for a.bc x 10Cl m into the blank . do no include units . do not include a sign A B 6.40 m X