\[\frac{a - b}{\text{ac}}\ \ \ \ \ \ при\ \ \ \ a = - 4;\ \ \ b = - 6;\ \ \ c = 5:\]
\[\frac{- 4 - ( - 6)}{- 4 \cdot 5} = \frac{- 4 + 6}{- 20} = \frac{2}{- 20} = - \frac{1}{10} = - 0,1.\]
\[Ответ:\ - 0,1.\]