Finding Angle Between Lines represented by Homogenous Equations

I am trying to find angle between two lines represented by a homogeneous equation

The equation is : $7x^2 + 4xy + 4y^2 = 0$

When i use the standard formula

$\theta = \arctan \frac {2 \sqrt {h^2 - ab}} {a + b}$

But $h^2 - ab$ is negeative and i cannot find the angle.

Any other method to find the angle between them?

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–  lab bhattacharjee Nov 30 '12 at 17:55
That post is now deleted. :) This is my actual question. ( And looks easier too ) @labbhattacharjee –  cipher Nov 30 '12 at 17:57
So, ain't anyone giving answers? –  cipher Nov 30 '12 at 17:59
the site somehow defines my problem but i could not get it. Can you please explain it in a clear way @labbhattacharjee . Please –  cipher Dec 1 '12 at 15:32

For a homogeneous equation to be a pair of straight line (passing through origin), $h^2 > ab$ is a must