If x + 1/x = 13 , then 3x ( x^2 - 1 ) equal to?

Answer: Option C

$$\begin{array}{rl}& \text{Given, }x+\frac{1}{x}=\sqrt{13}\\ & \text{then ,}\frac{3x}{\left(x^2-1\right)}\\ & =\frac{3}{x-\frac{1}{x}}.............(i)\\ & \text{Now, }x+\frac{1}{x}=\sqrt{13}\\ & \text{On squaring both side}\\ & =x^2+\frac{1}{x^2}\\ & =13-2\\ & =11\\ & =x^2+\frac{1}{x^2}-2\\ & =11-2\\ & =9\\ & \Rightarrow {\left(x-\frac{1}{x}\right)}^2=9\\ & \Rightarrow {\left(x-\frac{1}{x}\right)}^2=3^2\\ & \Rightarrow x-\frac{1}{x}=3\\ & \text{Put this value in equation (i)}\\ & \Rightarrow \frac{3}{x-\frac{1}{x}}\\ & \Rightarrow \frac{3}{3}\\ & \Rightarrow 1\end{array}$$