Smallest Acute angle.

Find the acute angle between the pair of lines given below,

$\left( l^{ 2 }\quad -\quad 3m^{ 2 } \right) x^{ 2 }\quad +\quad 81mxy\quad +\quad \left( m^{ 2 }\quad -\quad 3l^{ 2 } \right) y^{ 2 }\quad =\quad 0$