\[f(x) = 0:\]
\[\frac{1}{3}x^{2} + 2x = 0\]
\[x \cdot \left( \frac{1}{3}x + 2 \right) = 0\]
\[x = 0;\ \ x = - 6.\]
\[f(x) = 0\ \Longrightarrow при\ x = 0;\ \ x = - 6.\]