Calculus 8th Edition

$y'=3e^{\cosh 3x}\sinh (3x)$
Given: $y=e^{\cosh 3x}$ Now, $y'=\dfrac{d}{dx}[e^{\cosh 3x}]$ Apply chain rule. $y'=(\dfrac{de^{\cosh 3x}}{d\cosh 3x})(\dfrac{d\cosh 3x}{d3x}) (\dfrac{d(3x)}{dx})$ or, $=e^{\cosh 3x}(\sinh 3x)(3)$ Thus $y'=3e^{\cosh 3x}\sinh (3x)$