width = w
lenght = l
w -15 = l ------the width of a rectangle is 15 less than its length
perimeter = p = 2(l + w)
2(l+w) = 210
l + w = 210 /2
l + w = 105
since l = w -15 as i showed earlier, I'll substitute in (w - 15) for l
l + w = 105
w - 15 + w = 105
2w - 15 = 105
2w = 105 + 15
2w = 120
w = 120/2
w = 60