Question 55585
For a rectangle, the perimeter, P = 2L+2W = 84 ft.
But, L = 2W Substitute this for L in the above equation.
84 = 2(2W) + 2W Simplify and solve for W.
84 = 6W Divide both sides by 6.
14 = W
L = 2W 
L = 2(14)
L = 28

The length is 28 ft.