## Precalculus (6th Edition) Blitzer

$cot(2\theta)=\frac{A-C}{B}$
To eliminate the $xy$ term, we need to rotate the original axes by an angle $\theta$, which satisfies the equation $cot(2\theta)=\frac{A-C}{B}$, where $A,B,C$ can be obtained from the original equation.