Answer
$f^{\prime}(x)=3\cosh(3x)$
Work Step by Step
Apply Th.18
Let $u$ be a differentiable function of $x$.
$\displaystyle \frac{d}{dx}[\sinh u]=(\cosh u)u^{\prime}$
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$\displaystyle \frac{d}{dx}[\sinh(3x)]=\cosh(3x)\cdot 3$
$f^{\prime}(x)=3\cosh(3x)$
