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