I am taking math 124 at UW. The question is: A ladder 25 ft long is leaning against the wall of a building. Initially, the foot of the ladder is 7 ft from the wall. The foot of the ladder begins to slide at a rate of 2 ft/sec, causing the top of the ladder to slide down the wall. The location of the ladder at time t seconds is given by the parametric equation (7+2t, 0). I am stuck on part (d): Find a time interval[a,9] so that the average velocity of the top of the ladder on this time interval is -20 ft/sec i.e. a= ? I found the formula for y(t)= sqrt[25^2-(7+2t)^2] from part (a) The location of the top of the ladder will be given by parametric equations (0,y(t)). But I don't understand part d?