The perimeter of a rectangle is 24 feet. the length is 3 feet less than twice the width. the width is ?

Perimeter of a rectangle is twice the length plus twice the width or 

2W + 2L = Perimeter
where W = width and L = length

Per the description the length = 2w - 3

2W + 2(2W - 3) = 24
2W + 4W - 6 = 24
6W - 6 = 24
6W = 30
W = 30 / 6
W = 5

So your width is 5 feet. 

Plug in this result to the length equation:
2(5) - 3
10 - 3
7 = L

Your length is 7 feet. 

8doors on the second floor