\(P=\left(x-1\right)^3-6x+4=\left(\sqrt[3]{2}+\sqrt[3]{4}\right)^3-6\left(1+\sqrt[3]{2}+\sqrt[3]{4}\right)+4\)
\(=\left(2+4+3.\sqrt[3]{2}\sqrt[3]{4}\left(\sqrt[3]{2}+\sqrt[3]{4}\right)\right)-6\left(\sqrt[3]{2}+\sqrt[4]{4}\right)-6+4\)
\(=6+3.\sqrt[3]{8}\left(\sqrt[3]{2}+\sqrt[3]{4}\right)-6\left(\sqrt[3]{2}+\sqrt[3]{4}\right)-6+4\)
\(=4\)
\(=2^2\)