The diagonal and two adjacent sides of the square form an isosceles right triangle.
By the Pythagorean theorem
[tex]\begin{gathered} s^2+s^2=9^2 \\ 2s^2=81 \\ s^2=\frac{81}{2} \\ s=\sqrt[]{\frac{81}{2}} \\ s=\frac{9}{\sqrt[]{2}} \\ s=6.36\text{ cm} \end{gathered}[/tex]