24. If $x+\sqrt{x y}+y=9$ and $x^{2}+x y+y^{2}=27$, find the value of $x-\sqrt{x y}+y$.
24. Ans: 3 . Since $(x+\sqrt{x y}+y)(x-\sqrt{x y}+y)=(x+y)^{2}-(\sqrt{x y})^{2}=x^{2}+x y+y^{2}$, we have $9 \cdot(x-\sqrt{x y}+y)=27$. The result follows.