Simple...
you have: \(-\frac{6}{7} + \frac{6}{21}\)
You must find a common denominator first-->>
\(- \frac{6}{7} * \frac{3}{3} = -\frac{18}{21}\)
Now...
\(- \frac{18}{21} + \frac{6}{21} = -\frac{12}{21}\)
\(= -\frac{12}{21}\)
Simplify it-->>\(-\frac{4}{7}\)
Thus, your answer.
