\(\dfrac{49}{21}=\dfrac{7}{3}\)

\(\dfrac{12}{42}=\dfrac{2}{7}\)

16cm

\(\dfrac{180}{120}=\dfrac{36}{24}=\dfrac{9}{6}=\dfrac{3}{2}\)

a,\(x=-\dfrac{5}{6}+\dfrac{7}{12}\)
  \(x=-\dfrac{1}{4}\)
b,\(\dfrac{3}{8}:x=\dfrac{3}{8}-\left(-\dfrac{11}{12}\right)\)
   \(\dfrac{3}{8}:x=\dfrac{31}{24}\)
         \(x=\dfrac{3}{8}:\dfrac{31}{24}\)
        \(x=\dfrac{9}{31}\)
c,\(\dfrac{2}{3}.\left(x+\dfrac{5}{6}\right)=-\dfrac{5}{9}-\dfrac{1}{3}\)
   \(\dfrac{2}{3}.\left(x+\dfrac{5}{6}\right)=-\dfrac{8}{9}\)
   \(x+\dfrac{5}{6}=-\dfrac{8}{9}:\dfrac{2}{3}\)
   \(x+\dfrac{5}{6}=-\dfrac{4}{3}\)
   \(x=-\dfrac{4}{3}-\dfrac{5}{6}\)
   \(x=\dfrac{1}{2}\)