\[x² - 5x + k = 0;\ \ \ x_{1} = - 3\]
\[x_{1} + x_{2} = 5 \Longrightarrow - 3 + x_{1} = 5 \Longrightarrow\]
\[\Longrightarrow x_{2} = 8\]
\[x_{1} \cdot x_{2} = k \Longrightarrow - 3 \cdot x_{2} = k \Longrightarrow\]
\[\Longrightarrow - 3 \cdot 8 = k \Longrightarrow k = - 24.\]