Step 1 :
Factor out common factor $ \color{blue}{ 4m } $:
$$ 4mx^4-36my^4 = 4m ( x^4-9y^4 ) $$Step 2 :
Rewrite $ x^4-9y^4 $ as:
$$ \color{blue}{ x^4-9y^4 = (x^2)^2 - (3y^2)^2 } $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = x^2 $ and $ II = 3y^2 $ , we have:
$$ x^4-9y^4 = (x^2)^2 - (3y^2)^2 = ( x^2-3y^2 ) ( x^2+3y^2 ) $$