\[x = - 5\frac{5}{12}:\]
\[- 7,9 - x = - 7\frac{9}{10} - \left( - 5\frac{5}{12} \right) =\]
\[= - 7\frac{9}{10} + 5\frac{5}{12} =\]
\[= - \left( 7\frac{54}{60} - 5\frac{25}{60} \right) = - 2\frac{29}{60}.\]