Rewrite $ x^4-36y^6 $ as:
$$ \color{blue}{ x^4-36y^6 = (x^2)^2 - (6y^3)^2 } $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = x^2 $ and $ II = 6y^3 $ , we have:
$$ x^4-36y^6 = (x^2)^2 - (6y^3)^2 = ( x^2-6y^3 ) ( x^2+6y^3 ) $$