Rewrite $ 36c^2-121d^2 $ as:
$$ \color{blue}{ 36c^2-121d^2 = (6c)^2 - (11d)^2 } $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 6c $ and $ II = 11d $ , we have:
$$ 36c^2-121d^2 = (6c)^2 - (11d)^2 = ( 6c-11d ) ( 6c+11d ) $$