15. Suppose that $x$ and $y$ are non-zero real numbers such that
$$
\frac{x}{3}=y^{2} \quad \text { and } \frac{x}{9}=9 y \text {. }
$$

Find the value of $x+y$.
15. Ans: 2214 .
Eliminating $x$, we get $3 y^{2}=81 y$. Then $y=27$ since $y \neq 0$. Thus $x=2187$. The result follows.