A ladder 15 ft long leans against a vertical wall. If the lower end is being moved away from the wall at the rate of 4 /sec, how fast is the height of the top changing (this will be a negative rate) when the lower end is 9 feet from the wall? The height of the top is changing at a rate of B when the lower end is 9 feet from the wall. ' ' ' (Simplify your answer.)