We can start finding the distance of the sides of our quadrilatteral, lenghts of AB, BC, CD and DA, if it is a square, they all need to be the same
We can see that all this side belongs to a right triangle of cathetus 3 and 4, so we can find the length using Pitagoras Theorem, as such:
[tex]A\text{B}^2\text{ = 3}^2\text{ + 4}^2\text{ wich implies that AB = }\sqrt[]{25}\text{ so AB = 5}[/tex]
