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:

8m^2+18m+9=0
Multiply 2m by 4m to get 8m^2. Multiply 2m by 3 to get 6m. Multiply 3 by 4m to get 12m. Add the 6m and 12m to make 18m. Multiply 3 by 3 to get the integer 9. Set it all equal to 0.
Answer: The answer is 40 minutes.
Equation: If m is the total number of minutes it takes him to complete his homework, 30% of m is given by 0.3m. Since 30% was done in 12 minutes, we have 0.3m=12.
Right 9 spaces and up 13 spaces