\[\frac{\text{ab}}{a - c}\ \ \ при\ \ \ a = - 5;\ \ b = 6;\ \ c = 7:\]
\[\frac{- 5 \cdot 6}{- 5 - 7} = \frac{- 30}{- 12} = \frac{5}{2} = 2,5.\]
\[Ответ:2,5.\]