\[\boxed{\text{5.271.}}\]
\[\textbf{а)}\ 3 = \frac{7 \cdot 3}{7} = \frac{21}{7}\ \]
\[4 = \frac{7 \cdot 4}{7} = \frac{28}{7}\ \]
\[7 = \frac{7 \cdot 7}{7} = \frac{49}{7}\text{.\ }\]
\[\textbf{б)}\ 9 = \frac{6 \cdot 9}{6} = \frac{54}{6}\ \]
\[12 = \frac{6 \cdot 12}{6} = \frac{72}{6}\ \]
\[125 = \frac{6 \cdot 125}{6} = \frac{750}{6}\text{.\ }\]