\[\frac{3}{c} - \frac{2}{a - b} = \frac{3}{5} - \frac{2}{6} = \frac{3}{5} - \frac{1}{3} =\]
\[= \frac{3 \cdot 3 - 1 \cdot 5}{15} = \frac{9 - 5}{15} = \frac{4}{15}.\]