If  $A=\left[\begin{array}{ccc}1& 2& 1\\ 0& 1& -1\\ 3& -1& 1\end{array}\right] then A^3-3A^2-A+9I=$

$A^2=\left[\begin{array}{ccc}4& 3& 0\\ -3& 2& -2\\ 6& 4& 5\end{array}\right]$

$A^3=\left[\begin{array}{ccc}4& 11& 1\\ -9& -2& -7\\ 21& 11& 7\end{array}\right]$

$A^3-3A^2-A+9I=\left[\begin{array}{ccc}4& 11& 1\\ -9& -2& -7\\ 21& 11& 7\end{array}\right]-3\left[\begin{array}{ccc}4& 3& 0\\ -3& 2& -2\\ 6& 4& 5\end{array}\right]-\left[\begin{array}{ccc}1& 2& 1\\ 0& 1& -1\\ 3& -1& 1\end{array}\right]+9\left[\begin{array}{ccc}1& 0& 0\\ 0& 1& 0\\ 0& 0& 1\end{array}\right]$

$A^3-3A^2-A+9I=0$