I am trying to find angle between two lines represented by the following homogeneous equation: $ 7x^2 + 4xy + 4y^2 = 0.$
I tried to use the standard formula $ \theta = \arctan \left(\frac{2 \sqrt {h^2 - ab}}{a + b}\right),$ but here $h^2 - ab$ is negative and I cannot find the angle.
Is there any other method to find the angle between them?