Answer:

Step-by-step explanation:
Given:
- charges at the each of the three corner of a square,

- side of the square,

- charge at the remaining corner of the square,

Distance of the center of the square from each of the vertex of square:
Using Pythagoras theorem:



As there two equal like charges at an equal distance on the opposite ends of a diagonal so they will cancel out the effect of field due to each other.
Electric field at the center of the square due to
:



The charge on the opposite vertex will have the equal effect in the same direction which is towards the charge
. (refer the attached schematic)
So, the net electric field at the center:


