What is the value of: \[tan^{-1} \bigg( \frac {x}{y} \bigg) -tan^{-1} \frac {x-y}{x+y} \]

Question

\[ (A) \quad\frac{\pi}{2} \qquad (B) \quad \frac {\pi}{3}\qquad (C) \quad \frac {\pi}{4} \qquad (D)\quad \frac{-3\pi}{4} \]