$\begin{aligned} & \therefore x=3 \tan t \Rightarrow \frac{d x}{d t}=3 \sec ^2 t \\ & \text { Any } y=3 \sec t \Rightarrow \frac{d y}{d t}=3 \sec t \tan t \\ & \therefore \frac{d y}{d x}=\frac{\tan t}{\sec t}=\sin t \\ & \begin{aligned} \therefore \frac{d^2 y}{d x^2} & =\frac{d}{d x}(\sin t) \cdot \frac{d t}{d x} \\ & =\cos t \cdot \frac{1}{3 \sec ^2 t} \\ & =\frac{1}{3} \cos ^3 t \\ \therefore \frac{d^2 y}{d x^2}(\text { at } t & \left.=\frac{\pi}{4}\right)=\frac{1}{3} \cdot\left(\frac{1}{\sqrt{2}}\right)^3=\frac{1}{6 \sqrt{2}}\end{aligned}\end{aligned}$