To find the length of one side of a square when the perimeter is given, we can use the formula for the perimeter of a square:
\[ P = 4s \]
where \( P \) is the perimeter and \( s \) is the length of one side.
Given that the perimeter is 36 units, we can set up the equation:
\[ 4s = 36 \]
To find the length of one side, you can divide both sides of the equation by 4:
\[ s = \frac{36}{4} \]
Thus, the equation to find the length of one side is:
\[ s = 9 \]
So, each side of the square is 9 units long.