$\begin{array}{l}
\tan 6a = \tan \left( {4a + 2a} \right) = \dfrac{{\tan 4a + \tan 2a}}{{1 - \tan 4a.\tan 2a}}\\
\Rightarrow \tan 6a\left( {1 - \tan 4a.\tan 2a} \right) = \tan 4a + \tan 2a\\
\Leftrightarrow \tan 6a - \tan 4a - \tan 2a = \tan 2a.\tan 4a.\tan 6a
\end{array}$