If  $x=3\tan t$  and $y=3\sec t$   then the value of  $\frac{d^{2}y}{d{x}^2}$ at  $t=\frac{\pi }{4}$  is equal to 

$$\begin{gathered} \frac{dy}{dx}=\frac{dy/dt}{dx/dt}, \\ where \frac{dx}{dt}=3{\sec }^{2}t and \frac{dy}{dt}=3sect t\mathrm{an}t \\ \Rightarrow \frac{dy}{dx}=\sin t \\ Now \frac{d^{2}y}{d{x}^2}=\cos t.\frac{dt}{dx}=\frac{{\cos }^{3}t}{3} \\ {\frac{d^{2}y}{d{x}^2}}_{att=\frac{\pi }{4}}=\frac{1}{6\sqrt{2}} \end{gathered}$$