If you make the change of variables
$x = u \cos\theta - v\sin\theta$
$y = u \sin\theta + v\cos\theta$
where the angle $0\le \theta < \pi/2$ is chosen in order to eliminate the cross product term in
$3 x^2 + 7 xy + 3 y^2 = 80$
What is the angle you would use?
$\theta =$ .