What is the rule that you can use to calculate (√x - √y）ˆ2

$$\boxed{(a-b)^2=a^2-2ab+b^2}\\\\ so\\\\ (\sqrt{x}-\sqrt{y})^2\\\\ =(\sqrt{x})^2\;-\;2\sqrt{x}\sqrt{y}\;+\;(\sqrt{y})^2\\\\ =x-2\sqrt{xy}+y\\\\ =x+y-2\sqrt{xy}\\\\$$