P(\frac{X}{X+Y} \leq z)&=\int_{\frac{x}{x+y} \leq z}^{} 9e^{-3(x+y)}dx dy\\
	 &=\int_{0}^{\infty} \int_{\frac{1-z}{z}x}^{\infty}9e^{-3(x+y)}dy dx=z