\[\boxed{\text{335\ (335).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ \frac{x - 21}{x + 7} < 0\]
\[(x - 21)(x + 7) < 0\]
\[x \in ( - 7;21);\]
\[\textbf{б)}\ \frac{x + 4,7}{x - 7,2} > 0\]
\[(x + 4,7)(x - 7,2) > 0\]
\[x \in ( - \infty;\ - 4,7) \cup (7,2; + \infty).\]
\[\textbf{в)}\ \frac{6x + 1}{3 + x} > 0\]
\[(6x + 1)(x + 3) > 0\]
\[6 \cdot (x + 3)\left( x + \frac{1}{6} \right) > 0\]
\[x \in ( - \infty;\ - 3) \cup \left( - \frac{1}{6}; + \infty \right).\]
\[\textbf{г)}\ \frac{5x}{4x - 12} < 0\]
\[5x(4x - 12) < 0\]
\[20x(x - 3) < 0\]
\[x \in (0;3).\]