(b)
We have, $\cot{\theta - \tan{\theta = 2}}$
$\Rightarrow \ \cos^{2}{\theta - \sin^{2}{\theta = 2\sin{\theta\cos\theta}}}$
$\Rightarrow \ \cos{2\theta = \sin{2\theta}}$
$\Rightarrow \ \tan{2\theta = \tan{\frac{\pi}{4}\ \Rightarrow \ 2\theta = n\pi + \frac{\pi}{4}}}$
$\Rightarrow \ \theta = \frac{n\pi}{2} + \frac{\pi}{8}$