The square has 4 sides of the same measure.
If square SQUA has a side measuring 25 mm, it means side QU measures the same.
Also, if quadrilateral AQER is a square two, then its diagonals bisect each other, then QU is equal to UR.
Thus, QR measures:
![\begin{gathered} QR=QU+UR \\ QR=25\operatorname{mm}+25\operatorname{mm} \\ QR=50\operatorname{mm} \end{gathered}]()
The answer is 50 mm