Let me write it this way. His average speed is just going to be his change in distance over his change in time. And using the variables over here, we're saying y is distance, so this is the same thing as change in y over change in x from this point to that point. And this might look somewhat familiar to you from basic algebra. This is the slope between these two points. If I have a line that connects these two points, this is the slope of that line. The change in distance is this right over here.