Đáp án:
$\begin{array}{l}
\dfrac{{ - 3}}{7} + x = \dfrac{1}{3}\\
\Leftrightarrow x = \dfrac{1}{3} + \dfrac{3}{7}\\
\Leftrightarrow x = \dfrac{{16}}{{21}}\\
Vậy\,x = \dfrac{{16}}{{21}}\\
a)\dfrac{6}{7} + \left( {\dfrac{2}{{11}} - \dfrac{6}{7}} \right)\\
= \dfrac{6}{7} + \dfrac{2}{{11}} - \dfrac{6}{7}\\
= \dfrac{2}{{11}}\\
b)\dfrac{{ - 5}}{{11}} - \left( {\dfrac{7}{{19}} - \dfrac{5}{{31}}} \right)\\
= \dfrac{{ - 5}}{{11}} - \dfrac{7}{{19}} + \dfrac{5}{{31}}\\
= \dfrac{{ - 4287}}{{6479}}\\
c) - \left( {\dfrac{{11}}{{14}} - \dfrac{8}{{19}}} \right) - \left( {\dfrac{3}{{11}} - \dfrac{8}{{19}}} \right)\\
= \dfrac{{ - 11}}{{14}} + \dfrac{8}{{19}} - \dfrac{3}{{11}} + \dfrac{8}{{19}}\\
= \dfrac{{ - 11}}{{14}} - \dfrac{3}{{11}} + \dfrac{{16}}{{19}}\\
= \dfrac{{ - 633}}{{2926}}
\end{array}$
