\[- 8,3 - a = - 8,3 - \left( - 2\frac{4}{15} \right) =\]
\[= - \frac{83}{10} + \frac{34}{15} =\]
\[= - \left( \frac{249}{30} - \frac{68}{30} \right) = - \frac{181}{30} =\]
\[= - 6\frac{1}{30}.\]