jmc

We know that when $x+y=42$, $x=2y$. Substituting $2y$ in for $x$ in the first equation gives $3y=42$, or $y=14$. The value of $x$ is then $2(14)=28$. Since $x$ and $y$ are inversely proportional, the product $xy$ is constant. Let $xy=k$. When $x=28$ and $y=14$, $k=(28)(14)=392$. Therefore, when $x=-8$, $(-8)y=392$, giving $y=\boxed{-49}$.