\(\dfrac{-18}{24}+\dfrac{15}{-21}=\dfrac{-3}{4}+\dfrac{-5}{7}=\dfrac{-21}{28}+\dfrac{-20}{28}=\dfrac{-41}{28}\)
\(\dfrac{-18}{24}+\dfrac{15}{-21}=\dfrac{-3}{4}+\dfrac{-5}{7}=\dfrac{-21}{28}+\dfrac{-20}{28}=\dfrac{-21+\left(-20\right)}{28}=\dfrac{-41}{28}\)