\[\frac{3x + 1}{x - 3} > 0\]
\[\frac{3 \cdot \left( x - \left( - \frac{1}{3} \right) \right)}{x - 3} > 0\]
\[Ответ:\left( - \infty; - \frac{1}{3} \right) \cup (3; + \infty).\]