\[- 8,3 - a = - 8,3 - 6\frac{1}{3} =\]
\[= - \left( \frac{83}{10} + \frac{19}{3} \right) =\]
\[= - \frac{249 + 190}{30} =\]
\[= - \frac{439}{30} = - 14\frac{19}{30}\]