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