The width of a rectangle is 15 feet less than its length, and the perimeter is 210 feet. find the width:

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