Question 7063
Recall that the perimeter of a rectangle (we are assuming a rectangular-shaped garden) is 2(l + w), so half the perimeter would be just (l + w). You are told that half the perimeter is 18 feet, so (l + w) = 18.
Well, this eliminates all the choices except d.)
But to show the rest of it, you are also told that the garden is 8 feet longer than the width, or l = w + 8.  Rewriting this, you get: l - w = 8.

So answer d) l-w=8 and l+w=18 would be  your choice.