\[a = - 2,5;\ \ \ \ \ b = 1:\]
\[\frac{a + b}{3ab} = \frac{- 2,5 + 1}{3 \cdot ( - 2,5) \cdot 1} = \frac{- 1,5}{- 7,5} = \frac{15}{75} =\]
\[= \frac{3}{15} = \frac{1}{5}.\]