Question 41227
Let w be the width.  Then the length is 2w + 1.
Area  = length x width so we have
A = LW
21 = (2w + 1)w
21 = 2w^2 + w
2w^2 + w - 21 = 0
(2w + 7)(w - 3) = 0
w = -7/2 or w = 3
but since the width can't be negative,
w = 3 miles
and the length is 7 miles