$\ (\dfrac{-1}{6} + \dfrac{5}{-12}) + \dfrac{7}{12}$
$\ = \dfrac{-1}{6} + (\dfrac{-5}{12} + \dfrac{7}{12})$
$\ = \dfrac{-1}{6} + \dfrac{1}{6}$
$\ = 0$
$\ \dfrac{-4}{11} + (1 + \dfrac{7}{11})$
$\ = \dfrac{-4}{11} + \dfrac{7}{11} + 1$
$\ = \dfrac{3}{11} + 1 = \dfrac{14}{11}$
$\ \dfrac{3}{7} + (\dfrac{4}{7} + \dfrac{-2}{3})$
$\ = (\dfrac{3}{7} + \dfrac{4}{7}) + \dfrac{-2}{3}$
$\ = 1 + \dfrac{-2}{3} = \dfrac{1}{3}$