This problem is about the area of a square.
Notice that the mat has square form, this means its area is defined by
[tex]A=l^2[/tex]Where l is the length of each side.
Now, to find the length of the side, we just need to use the formula above and replace the area with its number 81
[tex]81=l^2[/tex]We solve for l
[tex]l=\sqrt{81}=9[/tex]