Answer:
40
Step-by-step explanation:
So you need to find out the numbers that make a square as big as this:
[tex]3600ft^{2}[/tex]
So you need to square root it
[tex]\sqrt{3600}[/tex]
You will get 60. Which means that each length is 60. Now subtract 60 by 20
[tex]60-20=40[/tex]
So your answer is 40!