$\left[\begin{array}{lll}1 & 2 & 3 \\ 0 & 1 & 0 \\ 1 & 1 & 0\end{array}\right]\left[\begin{array}{ccc}-1 & 1 & 0 \\ 0 & -1 & 1 \\ 2 & 3 & 4\end{array}\right]$

$\left[\begin{array}{lll}1(-1)+2(0)+3(2) & 1(1)+2(-1)+3(3) & 1(0)+2(1)+3(4) \\ 0(-1)+1(0)+0(2) & 0(1)+1(-1)+0(3) & 0(0)+1(1)+0(4) \\ 1(-1)+1(0)+0(2) & 1(1)+1(-1)+0(3) & 1(0)+1(1)+0(4)\end{array}\right]$

$\left[\begin{array}{ccc}5 & 8 & 14 \\ 0 & -1 & 1 \\ -1 & 0 & 1\end{array}\right]$

$\left[\begin{array}{ccc}-1 & 1 & 0 \\ 0 & -1 & 1 \\ 2 & 3 & 4\end{array}\right]\left[\begin{array}{lll}1 & 2 & 3 \\ 0 & 1 & 0 \\ 1 & 1 & 0\end{array}\right]$

$\left[\begin{array}{ccc}-1(1)+1(0)+0(1) & -1(2)+1(-1)+0(1) & -1(3)+1(0)+0(0) \\ 0(1)+(-1)(0)+1(1) & 0(2)+(-1)(1)+1(1) & 0(3)+(-1)(0)+1(0) \\ 2(1)+3(0)+4(1) & 2(2)+3(1)+4(1) & 2(3)+3(0)+4(0)\end{array}\right]$

$=\left[\begin{array}{ccc}-1 & -1 & -3 \\ 1 & 0 & 0 \\ 6 & 11 & 6\end{array}\right]$

$\therefore \left[ {\begin{array}{*{20}{l}}
  1&2&3 \\ 
  0&1&0 \\ 
  1&1&0 
\end{array}} \right]\left[ {\begin{array}{*{20}{c}}
  { – 1}&1&0 \\ 
  0&{ – 1}&1 \\ 
  2&3&4 
\end{array}} \right]$ $ \ne \left[ {\begin{array}{*{20}{c}}
  { – 1}&1&0 \\ 
  0&{ – 1}&1 \\ 
  2&3&4 
\end{array}} \right]\left[ {\begin{array}{*{20}{l}}
  1&2&3 \\ 
  0&1&0 \\ 
  1&1&0 
\end{array}} \right]$