a) \(\dfrac{-15}{16}\cdot\dfrac{8}{-25}=\dfrac{-15}{16}\cdot\dfrac{-8}{25}=\dfrac{-15\cdot\left(-8\right)}{16\cdot25}=\dfrac{120}{400}=\dfrac{3}{10}\)
b) \(\dfrac{-21}{24}\cdot\dfrac{8}{-14}=\dfrac{-21}{24}\cdot\dfrac{-8}{14}=\dfrac{-21\cdot\left(-8\right)}{24\cdot14}=\dfrac{168}{336}=\dfrac{1}{2}\)