let width = w let length = l
we know that w = 3l (1)
we know the perimeter is 56 in. the perimeter = 2(w+l) = 56 we devide both sides by 2 and we get
2(w+l)/2 = 56/2 ->> w+l = 28 (2) from (2) we have w=28-l (4) substituting where w=3l (because of (1))
we get 3l=28-l =>> 3l+l = 28 =>> 4l=28 ->> l=7 going back to (1) we have w=21
so length=7 inch and width = 21 inch.