Question 9284
Length = (2 x width) + 9

Let's say the width is given as 6 inches, then the Length = (2 x 6) + 9 inches
or the length = 21 inches.

If you are given the length as 29 inches, then  29 = (2 x W) + 9
Subtract 9 from each side of the equation:  20 = 2 x W
Divide each side 2, and you are left with 10 = W