Explanation:
l = length
w = width
l = w + 8
the whole perimeter (the whole frame around the picture) is 84 in = 2×l + 2×w
and when we use the first equation in the second we get :
84 = 2(w + 8) + 2w = 2w + 16 + 2w = 4w + 16
we divide both sides by 4 and get
21 = w + 4
we subtract 4 from both sides
17 = w
the width of the frame is 17 in.