Ellipse Question
Solution 1:
Using Derivation of the rotation formula, find $\theta$ to remove $xy$ from the equation.
Here $x=x'\cos \theta-y'\sin \theta$ and $y=x'\sin\theta +y'\cos\theta$
So, $x'=x\cos \theta+y\sin \theta,y'=y\cos \theta-x\sin \theta$.