The length of a rectangular garden is 8 feet longer than the width. 
L = W + 4 
Area of garden: W(W + 8) 
Area of sidewalk Plus garden: (W + 8)(W + 16) 
Subtract the areas to get the area of the sidewalk 
(W + 8)(W + 16) - W (W + 8) = 320 
W^2 + 24W + 128 - W^2 - 8W = 320 
16W = 192 
W = 12 
So L = 12 + 8 = 20