Answer:
14.1 units
Explanation:
In order to verify that the quadrilateral with diagonal (dotted line) is a square, we need to prove that the length of the hypotenuse of the triangle is 10 (it shares the side with the shape)
By Pythagorean theorem:

Hence, the quadrilateral is a square
=> ALL sides of the quadrilateral equal to 10 units
Again, by Pythagoren theorem:
