\[x² - 21x + 54 = 0\ \ \ \ \ \ \ \ и\ \ \ \ x_{1} = 3\ \]
\[x_{1} + x_{2} = 21 \Longrightarrow x_{2} + 3 = 21 \Longrightarrow\]
\[\Longrightarrow x_{2} = 18\]
\[x_{1} \cdot x_{2} = 54 \Longrightarrow x_{2} \cdot 3 = 54 \Longrightarrow\]
\[\Longrightarrow x_{2} = 18\]
\[Ответ:x_{2} = 18.\]