The number of words that I remember t days after studying is modeled by, so w of t, so this is the number of words I have in my head as a function of time is going to be equal to 80 times the one minus 0.1t squared, for t is between zero and 10, including the two boundaries. That's why we have brackets right over here. What is the rate of change of the number of words, of the number of known words per day two days after studying for the test? 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.