Width = w
Length = 3w - 7
Area = w (3w - 7) = 20
3w^2 - 7w = 20
3w^2 - 7w - 20 = 0
(3w + 5)(1w - 4) = 0
3w + 5 = 0
3w = -5
w = -5/3
That one doesn't work because you can't have a negative distance.
w - 4 = 0
w = 4
3w - 7 = 3*4 - 7 = 12 - 7 = 5
The width is 4 and the length is 5.