Hello..

First try to visualise the question as it is mentioned ABCD is a square of 5 metre length. Now we will try to find the length of diagonal of given square that will be:-
Multiplication of Square root of 2 and side.

Now from basic physics of electrostatic we know that opposite charge attract each other and charge of same magnitude at same distance will apply same electric field on each other.

Therefore resultant electric field between A and C will be cancel and hence only charge at D will apply net electric field on B.

Kq1/r^2 =(k *50)/(sqrt2*5)^2 =k 

Hence resultant electric field at B will be equal to value of constant K .

For any further queries feel free to comment.