Well I am assuming this is a rectangle. in a rectangle:
perimeter = 2 * width + 2 * length
We are told that:
length = width + 30
We can substitute out known value for length into the top equation:
perimeter = 2 * width + 2 * length
perimeter = 2 * width + 2 * (width + 30)
We are told that:
perimeter = 140
We can also sub that in:
perimeter = 2 * width + 2 * (width + 30)
(140) = 2 * width + 2 * (width + 30)
(140) = 2w + 2w + 60
80 = 4w
20 = w
We know width and we can sub in for our 2nd equation we made above:
length = width + 30
length = 20 + 30
l = 50
We can check by plugging in our answers to the top equation:
perimeter = 2 * width + 2 * length
140 = 2 * 20 + 2 * 50
140 = 140
it is correct