\[\boxed{\text{5.4}\text{3}\text{6.}}\]
\[\textbf{а)}\ \frac{3}{7}\ и\ \frac{5}{14}\]
\[\frac{3}{7} = \frac{6}{14};\ \ \ \ \frac{5}{14}\text{.\ }\]
\[\textbf{б)}\ \frac{3}{18}\ и\ \frac{7}{12}\ \]
\[\frac{3}{18} = \frac{6}{36};\ \ \ \ \frac{7}{12} = \frac{21}{36}\text{.\ \ }\]