This skill has been moved. Please see Chain Rule Core.
The chain rule for differentiating composite functions is covered in:
If $y = f(g(x))$, then:
$$\frac{dy}{dx} = f'(g(x)) \cdot g'(x)$$
Or in Leibniz notation, if $y = f(u)$ and $u = g(x)$:
$$\frac{dy}{dx} = \frac{dy}{du} \cdot \frac{du}{dx}$$