Answer:
Equation:  or 
Solution: 
Step-by-step explanation:
The formula that is used to calculate the perimeter of  a square is:
Where "s" is the side lenght the square.
Knowing that Xin used  20 yards of fencing to build the walls of a square chicken coop (in which the lenght in yards of each wall is represented with "x"), you can identify that:
Then, you can susbtitute values into the formula :
Finally, you must solve for "x" in order to find its value.
This is: