Perimeter of a rectangle = 2(L + B)
Let the length be x
Since the width is 15 ft. shorter than the length, = x - 15
90 = 2 (x + x -15)
90 = 2 (2x - 15)
open the brackets
90 = 4x - 30
collect like terms
90 + 30 = 4x
120 = 4x
divide both sides by 4
120/4 = 4x/4
30 = x
therefore, the length is 30 and the width is 30 - 15 = 15