Electric field intensity at centre due to charge Q having Q at opposite corner will be cancelled by electric field intensity of that opposite charge.
And electric field intensity of opposite Q and -2Q are in same direction.
distance between centre and charge Q = disstance between centre and charge -2Q = (5(21/2)/2)*10-2m
r =3.54*10-2m
So net electric field at centre
E=3KQ/r2
putting values of K Q and r
E = 0.215 N/C.
Hope it clears.
