The side length of the square with area of 81 cm² is :
↬ 9 cm
The side of the square with the perimeter of 8 m is :
↬ 2 m
Solution:
We're given the area of a square : 81 cm².
To find one of its sides, we need to take the square root of that.
So we have:

We know that if we take the square root of 81, we will get two solutions: 9 and -9; however we cannot use -9, because having a negative side length is impossible.
So the side length is 9 cm.
__________________________
To find the side length when given the perimeter, we divide it by 4, because the formula for a square's perimeter is P = 4a.
So if we rearrange that for a, we will get : a = P/4.
where a = side length
Solving,
Hence, the length of each side is 4 m.