Matrix polynomials through eigenvalues

Let $P$ be a $3\times 3$ matrix having characteristic roots $\lambda_{1}=-\dfrac{2}{3}$, $\lambda_{2}=0$ and $\lambda_{3}=1$. Define $Q=3P^{3}-P^{2}-P+I_{3}$ and $R=3P^{3}-2P$. If $\alpha=\det(Q)$ and $\beta=\operatorname{trace}(R)$, then $\alpha+\beta$ equals __________ (round off to two decimal places)