A-I=\left ( \begin{array}{rlc} -3 & 5 & 7 \\ 1 & -1 & -1 \\ -1 & 1 & 1 \end{array} \right) \sim \left( \begin{array}{rlc} -3 & 5 & 7 \\ 1 & -1 & -1 \\ 0 & 0 & 0 \end{array} \right) \sim \left( \begin{array}{rlc} 0 & 2 & 4 \\ 1 & -1 & -1 \\ 0 & 0 & 0 \end{array} \right)