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Q$2)$ Determine by inspection whether the matrix is invertible

$19.\left[\begin{array}{ccc}0& 6& -1\\ 0& 7& -4\\ 0& 0& -2\end{array}\right]$

$20.\left[\begin{array}{ccc}-1& 2& 4\\ 0& 3& 0\\ 0& 0& 5\end{array}\right]$

$21.\left[\begin{array}{cccc}1& 0& 0& 0\\ 2& -5& 0& 0\\ 4& -3& 4& 0\\ 1& -2& 1& 3\end{array}\right]$

$22.\left[\begin{array}{cccc}2& 0& 0& 0\\ -3& -1& 0& 0\\ -4& -6& 0& 0\\ 0& 3& 8& -5\end{array}\right]$