\(\frac{6}{7}-\left(x-\frac{1}{2}\right)=\frac{5}{6}\)
\(x-\frac{1}{2}=\frac{6}{7}-\frac{5}{6}=\frac{36}{42}-\frac{35}{42}\)
\(x-\frac{1}{2}=\frac{1}{42}\)
\(x=\frac{1}{42}+\frac{1}{2}=\frac{1}{42}+\frac{21}{42}=\frac{22}{42}=\frac{11}{21}\)
=> 21x=\(21\cdot\frac{11}{21}=1\cdot\frac{11}{1}=11\)
Vậy 21x=11