\[\left\{ \begin{matrix} 7x - 21 < 0 \\ 5x + 10 > 0 \\ \end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix} 7x < 21\ \ \ \\ 5x > - 10 \\ \end{matrix} \right.\ \Longrightarrow \left\{ \begin{matrix} x < 3\ \ \ \\ x > - 2 \\ \end{matrix} \right.\ \]
\[Ответ:\ x \in ( - 2;3).\]