Rewrite $ 36y^6-x^2 $ as:
$$ \color{blue}{ 36y^6-x^2 = (6y^3)^2 - (x)^2 } $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 6y^3 $ and $ II = x $ , we have:
$$ 36y^6-x^2 = (6y^3)^2 - (x)^2 = ( 6y^3-x ) ( 6y^3+x ) $$