\[x² + kx + 18 = 0;\ \ \ x_{1} = - 3\]
\[x_{1} + x_{2} = - k \Longrightarrow - 3 + x_{2} = k \Longrightarrow\]
\[\Longrightarrow - 3 - 6 = - k \Longrightarrow k = 9\]
\[x_{1} \cdot x_{2} = 18 \Longrightarrow - 3 \cdot x_{1} =\]
\[= 18 \Longrightarrow x_{2} = - 6.\]