Straight Lines #1

The combined equation of the lines $L_1$ and $L_2$ is $2x^2+6xy+y^2=0$ and that of the lines $L_3$ and $L_4$ is $4x^2+18xy+y^2=0$ . If the angles between $L_1$ and $L_3$ is $\theta$ , then what is the angle between $L_2$ and $L_4$ ?