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