The area of a square is given by:
[tex]A=l^2[/tex]Where:
l = length of one of its sides:
Since the area is 81, and the length of one of its sides is x, then:
[tex]81=x^2[/tex]Take the square root of both sides:
[tex]\begin{gathered} \sqrt[]{81}=\sqrt[]{x^2} \\ 9=x \\ x=9 \end{gathered}[/tex]