\[f(x) = \sqrt[3]{x}\]
\[f\left( x^{2} \right) - 5f(x) + 6 = 0\]
\[\sqrt[3]{x^{2}} - 5\sqrt[3]{x} + 6 = 0;\ \ \ \ \ \ \ t = \sqrt[3]{x}\]
\[t^{2} - 5t + 6 = 0\]
\[D = 25 - 24 = 1\]
\[t_{1} = \frac{5 + 1}{2} = 3;\text{\ \ \ \ \ \ }\]
\[t_{2} = \frac{5 - 1}{2} = 2\]
\[\left( \sqrt[3]{x} \right)^{3} = (3)^{3}\text{\ \ \ \ \ \ \ \ \ \ }\left( \sqrt[3]{x} \right)^{3} = (2)^{3}\]
\[x = 27\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ x = 8\]
\[Ответ:\ \ x = 8;\ \ x = 27.\]