And so the rate of change of the base with respect to time is going to be negative 13. They gave us that. Now A prime of t, this is the rate of change of the area at time t sub zero. Did they give us this? Well, they ask us that. What is the rate of change of the area, A of t of the triangle at that instant? So this is what we actually need to figure out, but they haven't given it to us, otherwise there's no problem to solve.