According to my textbook, the second derivative of
\begin{equation*} y^{2}+xy-x^{2}=9 \end{equation*}
is
\begin{equation*} \frac{90}{(2y+x)^{3}}. \end{equation*}
The problem states "Express $\frac{d^{2}y}{dx^{2}}$ in terms of $x$ and $y$." I've tried for two days straight now, and I can't get that answer. I am convinced the book has a typo.