Answer
$$ y'= \frac{3(\tan^{-1}x)^2}{1+x^2}.$$
Work Step by Step
Since $ y=(\tan^{-1}x)^3$, then the derivative is given by
$$ y'=3(\tan^{-1}x)^2(\tan^{-1}x)'=\frac{3(\tan^{-1}x)^2}{1+x^2}.$$
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