\(H=\left(-\dfrac{5}{24}+0,75+\dfrac{7}{12}\right)\div\left(-\dfrac{21}{8}\right)\)
\(H=\left(-\dfrac{5}{24}+\dfrac{3}{4}+\dfrac{7}{12}\right).\left(-\dfrac{8}{21}\right)\)
\(H=\left(-\dfrac{5}{24}+\dfrac{18}{24}+\dfrac{14}{24}\right).\left(-\dfrac{8}{21}\right)\)
\(H=\dfrac{-5+18+14}{24}.\left(-\dfrac{8}{21}\right)\)
\(H=\dfrac{27}{24}\left(-\dfrac{8}{21}\right)\)
\(H=-\dfrac{3}{7}\)
Vậy \(H=-\dfrac{3}{7}\)