If you take the perimeter, 24, and cut it in half, you are left with the sum of two sides of the rectangle. This is easier to manage. Now, make a list of numbers whose sum is 12. I got 1+11, 2+10, 3+9, 4+8, 5+7, 6+6
The other part of the problem said that the length is 3 times its width. look at the pairs of numbers, and is there a set the has one number that is three times larger than the other number in that same set? Yes, 3 and 9. (9 is three times larger than 3)
So, the length of the rectangle is 9 inches and the width is 3 inches.