[tex] \dfrac{\sqrt{x}}{3\sqrt{x}-\sqrt{y}}=\dfrac{\sqrt{x}}{3\sqrt{x}-\sqrt{y}}\cdot\dfrac{3\sqrt{x}+\sqrt{y}}{3\sqrt{x}+\sqrt{y}}=\dfrac{3(\sqrt{x})^2+(\sqrt{x})(\sqrt{y})}{(3\sqrt{x})^2-(\sqrt{y})^2}\\\\=\dfrac{3x+\sqrt{xy}}{9x-y}\\\\Used:\\(a-b)(a+b)=a^2-b^2\\\\(\sqrt{a})^2=a\ for\ a\geq0\\\\\sqrt{a}\cdot\sqrt{b}=\sqrt{ab}\ for\ a,b\geq0 [/tex]
