The area of any rectangle is xy where x and y are the two dimensions. A square is just a special rectangle where x=y, which are equal to a side length. So the area of a square is always just:
A=s^2, where s is a side length.
In this case we are told that the area is 81m^2 so:
s^2=81
s=√81
s=9m
So the square has sides of length 9 meters.
