The number of values of $x$ satisfying $\tan ^ { - 1 } ( 4 x ) + \tan ^ { - 1 } ( 6 x ) = \frac { \pi } { 6 }$ and $\mathrm { x } \in \left[ - \frac { 1 } { 2 \sqrt { 6 } } , \frac { 1 } { 2 \sqrt { 6 } } \right]$ is (A) 0 (B) 1 (C) 2 (D) 3