$\begin{array}{rcl}\left[\begin{array}{ccc}2& 0& 7\\ 0& 1& 0\\ 1& -2& 1\end{array}\right] \left[\begin{array}{ccc}-\mathrm{\lambda }& 14\mathrm{\lambda }& 7\mathrm{\lambda }\\ 0& 1& 0\\ \mathrm{\lambda }& -4\mathrm{\lambda }& -2\mathrm{\lambda }\end{array}\right]& =& \left[\begin{array}{ccc}1& 0& 0\\ 0& 1& 0\\ 0& 0& 1\end{array}\right]\\ & \Rightarrow & -2\mathrm{\lambda }+7\mathrm{\lambda }=1\\ 5\mathrm{\lambda }& =& 1\\ \mathrm{\lambda }& =& \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$5$}\right. \end{array}$