If $\mathrm{f}:\mathrm{R}\to \mathrm{R}$ is defined in $\mathrm{f}\left(\mathrm{x}\right)=\left\{\begin{array}{cc}\frac{\cos 3\mathrm{x}-\cos \mathrm{x}}{{\mathrm{x}}^2}& \text{ for }\ne 0\\ \mathrm{\lambda }& \text{ for }\mathrm{x}=0\end{array}\right.$ and if f is continuous at x = 0 , then $\left|\mathrm{\lambda }\right|=$