\[\frac{a - c}{\text{ab}}\ \ \ \ при\ \ \ a = - 2;\ \ \ b = - 1;\ \ c = 3:\]
\[\frac{- 2 - 3}{- 2 \cdot ( - 1)} = \frac{- 5}{2} = - 2,5.\]
\[Ответ:\ - 2,5.\]