Answer
$f''(\theta)=6\sec^2{\theta}\tan{\theta}.$
Work Step by Step
Using the Power Rule:
$f'(\theta)=3\sec^2{\theta}.$
Using the Chain Rule with $u=\sec{\theta}\rightarrow\dfrac{du}{dx}=\sec{\theta}\tan{\theta}$
$f(u)=3u^2\rightarrow f'(u)=6u$
$f''(\theta)=6(\sec{\theta})(\sec{\theta}\tan{\theta})=6\sec^2{\theta}\tan{\theta}$
