Find the value of $\tan^{-1} (\frac{x}{y}) - \tan^{-1}(\frac{x-y}{x+y})$

Question

$(A) \; \frac{\pi}{4}$
$(B)\; \frac{\pi}{2}$
$(C) \; \frac{\pi}{3}$
$(D) \; \frac{\pi}{6}$