\[x² - 15x + 36 = 0\ \]
\[x_{1} + x_{2} = 15\]
\[x_{1} \cdot x_{2} = 36 \Longrightarrow x_{1} = 12;\ \ x_{2} = 3\]
\[Ответ:x_{1} = 12;\ \ x_{2} = 3.\]