Below is a labeled rectangle:
length = w + 20
we know length is 36 so we plug that in:
36 = w + 20
We need to isolate w and to do that we must subtract 20 from both sides:
(36 - 20) = w + (20 - 20)
16 = w + 0
w = 16
width is 16 feet
Hope this helped!
