$\begin{align}  & \left| A \right|=6-(\left| A \right|-1)(\left| A \right|+2)\Rightarrow \left| A \right|=6-{{\left| A \right|}^{2}}-\left| A \right|+2 \\  & \Rightarrow {{\left| A \right|}^{2}}+2\left| A \right|-8=0\Rightarrow (\left| A \right|+4)(\left| A \right|-2)=0 \\  & \Rightarrow \left\{ \begin{matrix}   \left| A \right|=2  \\   \left| A \right|=-4  \\\end{matrix} \right. \\ \end{align}$