Complete Question
The complete question is shown on the first uploaded image
Answer:
The electric potential is

Step-by-step explanation:
From the question we are told that
The charge on each point is +Q
The length of the diagonal is 2a
Generally the electric potential is mathematically represented as

Here r is the distance from each vertices of the square to the center P which is evaluated as

So the potential at the center due to one charge is evaluated as

Given that there are four charge the net electric potential at the center P will be
