1. The problem is to understand why the differential $du$ equals $2x$ in the context of calculus. 2. Typically, $du$ represents the differential of a function $u$ with respect to $x$. If $u = x^2$, then to find $du$, we use the derivative formula: $$du = \frac{du}{dx} dx$$ 3. The derivative of $u = x^2$ with respect to $x$ is: $$\frac{du}{dx} = 2x$$ 4. Therefore, the differential $du$ is: $$du = 2x \, dx$$ 5. In many contexts, especially when $dx$ is implied or considered as a small change in $x$, we write $du = 2x$ to indicate the rate of change of $u$ with respect to $x$. 6. So, $du = 2x$ comes from differentiating $u = x^2$ and represents how $u$ changes as $x$ changes.