So what does this simplify to? Well, this right over here, this is the definition, and we're assuming, in order for this to even be true, we have to assume that u and y are differentiable at x. So we assume, in order for this to be true, we're assuming, we're assuming y, comma, u are differentiable, are differentiable, are differentiable at x. And remember, also, if they're differentiable at x, that means they're continuous at x. But if u is differentiable at x, then this limit exists, and this is the derivative of, this is u prime of x, or du dx. So this right over here, we can rewrite as du dx. I think you see where this is going.