So the slope over here might be 23 whatever miles per hour. But the instantaneous, his fastest point in this 9.58 seconds, is closer to 30 miles per hour. But you see, it's not a trivial thing to do. You could say, OK, let me try to approximate the slope right over here. And you could do that by saying, OK, well, what is the change in y over the change of x right around this? So you could say, well, let me take some change of x and figure out what the change of y is around it, or as we go past that. So you get that.