$\Rightarrow P = aP ^{ T }+( a -1) I$
$\Rightarrow P ^{ T }- P = a \left( P - P ^{ T }\right)$
$\Rightarrow P = P ^{ T } \text {, as } a \neq-1$
$\text { Now, } P = aP +( a -1) I$
$\Rightarrow P =- I \Rightarrow| P |=1$
$\Rightarrow|\operatorname{Adj} P |=1$