In an inverse variation, the product $x y$ is constant. If $\left(x_{1}, y_{1}\right)$ and $\left(x_{2}, y_{2}\right)$ are solutions of $x y=k,$ then $x_{1} y_{1}=x_{2} y_{2} .$ Use this equation to find the missing value.
Find $x_{2}$ when $x_{1}=9, y_{1}=-3,$ and $y_{2}=12$.