Answer
$$ y'= 6(4x^3-9) (x^4-9x)^5 .$$
Work Step by Step
Recall that $(x^n)'=nx^{n-1}$
Since $ y=(x^4-9x)^6$, then by the chain rule the derivative $ y'$ is given by
$$ y'=6 (x^4-9x)^5(4x^3-9)=6(4x^3-9 )(x^4-9x)^5 .$$
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