So what would be the change in position between time a and between time b? Well, at time b, we are at s of b. We are at s of b position. And at time a, we were at s of a position. So the change in position between time a and time b, let me write this down, the change in position between, and this might be obvious to you, but I'll write it down, between times a and b is going to be equal to s of b, this position, s of b minus this position, minus s of a. So nothing earth shattering so far. But now let's think about what happens if we take the derivative of this function right over here.