So for example, how fast is y changing with respect to x exactly at that point, exactly when x is equal to that value? Let's call it x one. Well, one way you could think about it is what if we could draw a tangent line to this point, a line that just touches the graph right over there? And we can calculate the slope of that line. Well, that should be the rate of change at that point, the instantaneous rate of change. So in this case, the tangent line might look something like that. If we know the slope of this, well, then we could say that that's the instantaneous rate of change at that point.