To solve this question identify each of the variables. In this case both length and width.
Say length = x
Width = 2x + 8
Perimeter = 34
Perimeter = 2lengths + 2 widths
34 = 2(x) + 2(2x+8)
34 = 2x + 4x + 16
34 = 6x + 16
34 - 16 = 6x
18 = 6x
18/6 = x
X = 3 inches
The length of the rectangle is 3 inches.