Lets set up the relationship of the length to width:
l = 3w
Using this, we can make all the variables the same by substitution:
2l + 2w = perimeter
2l + 2w = 72
2(3w) + 2w = 72
6w + 2w = 72
8w = 72
w = 9
The width is 9, to the length must be 9*3, or 27.
Hope this helped!
