Find the Exact Value of the Expression $$\tan ^ { - 1 } \left[ \tan \left( \frac { 6 \pi } { 7 } \right) \right]$$

Multiple Choice

Find the exact value of the expression. Do not use a calculator.
- $$\tan ^ { - 1 } \left[ \tan \left( \frac { 6 \pi } { 7 } \right) \right]$$

A) $- \frac { 6 \pi } { 7 }$
B) $\frac { \pi } { 7 }$
C) $- \frac { \pi } { 7 }$
D) $\frac { 6 \pi } { 7 }$