\[\boxed{\text{5.\ 53}}\]
\[\textbf{а)}\ \frac{a}{- 5,8} = \frac{5,3}{2,9}\]
\[a = - \frac{5,8 \cdot 5,3}{2,9} = - 2 \cdot 5,3\]
\[a = - 10,6.\]
\[\textbf{б)}\ \frac{- 7\frac{1}{4}}{c} = \frac{4\frac{1}{7}}{1\frac{3}{7}}\]
\[4\frac{1}{7}c = - 7\frac{1}{4} \cdot 1\frac{3}{7}\]
\[\frac{29}{7}c = - \frac{29}{4} \cdot \frac{10}{7}\]
\[c = - \frac{29}{4} \cdot \frac{10}{7} \cdot \frac{7}{29} = - \frac{10}{4}\]
\[c = - 2,5.\]