In the given example:
[tex]\begin{gathered} u=4x^3-5 \\ f(u)=u^4 \\ \text{If we do a function composition then they will be the same} \\ f(x)=\big(4x^3-5\big)^4\rightarrow f(u)=u^4,\text{ note that }u=4x^3-5 \end{gathered}[/tex]Solve for each derivative of dy/du and du/dx
[tex]\begin{gathered} \frac{du}{dx}=3\cdot4x^{3-1}-0 \\ \frac{du}{dx}=12x^2 \\ \\ \frac{dy}{du}=4\cdot u^{4-1} \\ \frac{dy}{du}=4u^3,\text{ then substitute }u \\ \frac{dy}{du}=4(4x^3-5)^3 \\ \\ \text{Complete the chain rule} \\ \frac{dy}{dx}=\frac{dy}{du}\cdot\frac{du}{dx} \\ \frac{dy}{dx}=\big(4(4x^3-5)^3\big)\big(12x^2\big)\text{ or }\frac{dy}{dx}=48x^2(4x^3-5)^3 \\ \end{gathered}[/tex]