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.
- $\tan ^ { - 1 } \left[ \tan \left( \frac { 6 \pi } { 7 } \right) \right]$

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