So times the derivative of u with respect to x. And just to make clear, so you're familiar with the different notations you'll see in different textbooks, this is this right over here, just using different notations, and this is this right over here. So let's actually evaluate these things. You're probably tired of just talking in the abstract. So this is going to be equal to, this is going to be equal to, and I'm gonna write it out again, this is the derivative, instead of just writing v and u, I'm gonna write it, let me write it this way, this is going to be, I keep wanting to be using the wrong colors, this is going to be the derivative of, I'm gonna leave some space, times the derivative of something else with respect to something else. So we're gonna first take the derivative of v. Well v is cosine of x to the third power, cosine of x. We're gonna take the derivative of that with respect to u, which is just cosine of x, and we're gonna multiply that times the derivative of u, which is cosine of x, with respect to x, so this one we have good, we've seen this before.