And I encourage you to pause this video and try it on your own. So the key here is we come up with this equation for modeling how many words have retained in my brain every day after I first memorized them, after I got the 80 of them into my head. And that's this expression here. And they wanna know the rate of change two days after studying. Well, the rate of change, I can take the derivative of this with respect to time. So let's do that. So let's take the derivative, the derivative of the number of words I know with respect to time is going to be equal to, well, we have this 80 out front.