$P ^\alpha+ P ^\beta=\gamma I -29 P , P ^\alpha- P ^\beta=\delta I -13 P$
$P ^4=( I - P )^2= I -2 P + P ^2=2 I -3 P$
$P ^6=(2 I -3 P )( I - P )=5 I -8 P$
$P ^8=(2 I -3 P )^2=4 I -12 P +9( I - P )=13 I -21 P$
$P ^8+ P ^6=18 I -29 P$
$P ^8- P ^6=8 I -13 P$
$\alpha=8 ; \beta=6 ; \gamma=18, \delta=8$
$\alpha+\beta+\gamma-\delta=8+6+18-8=24$