Solve the Equation for Exact Solutions $6 \cos ^ { - 1 } x = \pi$

Solve the equation for exact solutions.
- $6 \cos ^ { - 1 } x = \pi$

A) $\left\{ \frac { \sqrt { 2 } } { 2 } \right\}$
B) $\left\{ \frac { \pi } { 6 } \right\}$
C) $\left\{ \frac { 1 } { 2 } \right\}$
D) $\left\{ \frac { \sqrt { 3 } } { 2 } \right\}$