It sounds like an easy question, but it's not straightforward. The trouble is that you can create many different rectangles with a perimeter of 18 feet. For example, a rectangle that is 3 by 6 would work, but so would a rectangle 4 by 5 or 1 by 8. There is no one right answer unless you are given more information.
Remember that the perimeter of a rectangle, using algebra, is 2W + 2L (where W is width and L is length). That means that for any value of L less than 9 feet, there is a corresponding value for W that makes the perimeter 18 feet.
Stephen La Rocque.>