\(a=\sqrt[3]{2}+2\)
\(P\left(a\right)=\left(2+\sqrt[3]{2}-3\right)\left(2+\sqrt[3]{2}-1\right)^3\)
\(=\left(\sqrt[3]{2}-1\right)\left(\sqrt[3]{2}+1\right)^3\)
\(=\left(\sqrt[3]{2}-1\right)\left(2+3\sqrt[3]{4}+3\sqrt[3]{2}+1\right)\)
\(=\left(\sqrt[3]{2}-1\right)\cdot3\cdot\left(\sqrt[3]{4}+\sqrt[3]{2}+1\right)\)
\(=3\cdot\left(2-1\right)=3\)