The area of the square is 81 square meters what is the length of one side of the square

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.