width = x and length = x + 5; Area of a rectangle = (length)x(width)

x(x+5) = 336

x^2 + 5x = 336

x^2 + 5x - 336 = 0

(x + 21)(x - 16) = 0

x + 21 = 0 OR x - 16 = 0

x = -21 OR x = 16

x cannot be -21, because that does not make sense for a length or a width. Therefore, x (the width) must be 16 ft. This makes the length (x + 5) 21 ft.