The highlinghter is the answer, but I don't know why. Please help me to solve and explain.

1 1. Twelve charges are arranged equally spaced around a square of side length 1.0 m, as shown in the diagram. What is the electric field strength at the centre of the square? -2.3 nC -1.5 nC 1.0 nc . . 1.7 nc 2.6 nc -0.9 nc 1.0 m -0.9 nc .2.6 nc 1.7 nc . -1.0 nC -1.5 nC -2.3 nc a) E = 0 N/C b) E = 36 N/C C) E = 72 N/C d) E =96 N/C 12 . An electric dipole consists of a particle with a charge of Q1 = +6.0 . 10- C at the origin and a particle with a charge of Q2 = -6.0 . 10-6 C on the x-axis at x = 3.0 . 10-3 m. Q1 Q2 R2 R3 What is the dipole moment ? a) 1.8 x 10-8 C.m, in the positive x-direction b) 1.8 x 10-8 C.m, in the negative x-direction c) 3.6 x 10-8 C.m, in the positive x-direction d) 3.6 x 10-8 C.m, in the negative x-direction