Answer
$y''$ = $2sec^{2}(\frac{x}{3})tan(\frac{x}{3})$
Work Step by Step
$y$ = $9tan(\frac{x}{3})$
$y'$ = $\frac{d}{dx}$[$9tan(\frac{x}{3})$]
$y'$ = $9sec^{2}(\frac{x}{3})$ $\frac{d}{dx}$$(\frac{x}{3})$
$y'$ = $[9sec^{2}(\frac{x}{3})]$ $[\frac{1}{3}]$
$y'$ = $3sec^{2}(\frac{x}{3})$
$y''$ = $\frac{d}{dx}$$3sec^{2}(\frac{x}{3})$
$y''$ = $3(2)sec(\frac{x}{3})$ $\frac{d}{dx}$$[sec(\frac{x}{3})]$
$y''$ = $[6sec(\frac{x}{3})]$$[sec(\frac{x}{3})tan(\frac{x}{3})]$ $\frac{d}{dx}$$(\frac{x}{3})$
$y''$ = $[6sec^{2}(\frac{x}{3})tan(\frac{x}{3})]$$[\frac{1}{3}]$
$y''$ = $2sec^{2}(\frac{x}{3})tan(\frac{x}{3})$
