If $A=\left[\begin{array}{l}1\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}2\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}3\\ -4\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}-5\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}6\end{array}\right]\text{, then }\left|A\right|$ is 

The given matrix is $A=\left[\begin{array}{ccc}1& 2& 3\\ -4& -5& 6\end{array}\right]$ Since, this matrix is not a square matrix. 

Therefore, its determinant does not exist.