a) \(\dfrac{-2}{7}.\dfrac{21}{8}=\dfrac{-2.21}{7.8}=\dfrac{-42}{56}=\dfrac{-3}{4}\)
b) \(0,24.\dfrac{-15}{4}=\dfrac{0,24.\left(-15\right)}{4}=\dfrac{-3,6}{4}=\dfrac{-9}{10}\)
c) \(\left(-2\right).\dfrac{\left(-7\right)}{12}=\dfrac{\left(-2\right)\left(-7\right)}{12}=\dfrac{14}{12}=\dfrac{7}{6}\)
d) \(\left(\dfrac{-3}{25}\right):6=\dfrac{\left(-3\right)}{25.6}=\dfrac{-3}{150}=\dfrac{-1}{50}\)