$A ^3=3 A ^2+\alpha A$
$A ^3=3(3 A +\alpha I )+\alpha A$
$A ^3=9 A +\alpha A +3 \alpha I$
$A ^4=(9+\alpha) A ^2+3 \alpha A$
$=(9+\alpha)(3 A +\alpha I )+3 \alpha A$
$= A (27+6 \alpha)+\alpha(9+\alpha)$
$\Rightarrow 27+6 \alpha=21 \Rightarrow \alpha=-1$
$\Rightarrow \beta=\alpha(9+\alpha)=-8$
વિધાન $2$: કોઇપણ શ્રેણિક $A$ માટે $\det \left( {{A^T}} \right) = {\rm{det}}\left( A \right)$ અને $\det \left( { - A} \right) = - {\rm{det}}\left( A \right)$ જયાં $\det \left( A \right) = A$ નો નિશ્રાયક.