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