\[(x - 3)^{4} - 5 \cdot (x - 3)^{2} + 4 = 0\]
\[Пусть\ \ t = (x - 3)^{2}:\]
\[t^{2} - 5t + 4 = 0\]
\[t_{1} + t_{2} = 5;\ \ t_{1} \cdot t_{2} = 4\]
\[t_{1} = 4,\ \ t_{2} = 1.\]