\[5a + 2b\]
\[При\ a = \frac{7}{15};\ \ b = - \frac{5}{6}:\]
\[5 \cdot \frac{7}{15} + 2 \cdot \left( - \frac{5}{6} \right) = \frac{7}{3} - \frac{5}{3} = \frac{2}{3}.\]