(c)
Let $x = \tan{\theta\ \ \Rightarrow \ \ \ \theta = \tan^{- 1}x}$
$\therefore\ \tan^{- 1}\left( \frac{3x - x^{3}}{1 - 3x^{2}} \right) - \tan^{- 1}\left( \frac{2x}{1 - x^{2}} \right)$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 3\tan^{- 1}{x - 2\tan^{- 1}x}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = \tan^{- 1}x$