Question 911226
Let x be the length of the field and y the width. The first condition in the question says that x= 2y-15. Then the perimeter gives us a linear equation: 
2y + 2(2y-15) = 330
This can be simplified to 6y-30 = 330 or 6y = 300 so y = 60. Then,  by x = 2y-15, x = 105. Now from area of a rectangle, x*y, we get the total field area to be: 6300. Since the defensive area is 5/21 the total area we conclude that it must be 1500 yards square.