If the pair of lines $6x^2+xy-y^2=0$  and $3x^2-axy-y^2=0$, $a>0$  have a common line then 2a =__________

 $6x^2+xy-y^2=(2x+y)(3x-y)$
Equation of Lines are 

  $\begin{array}{l}y=-2x \mathrm{...}\left(1\right)\\ y=3x \mathrm{...}\left(2\right)\end{array}$
Sub $y=-2x$   in
 $3x^2-axy-y^2=0$
$\Rightarrow 2a=1$