(2m-n)^2-121(m+n)^2
=(2m-n)^2-[11(m+n)]^2
=[2m-n+11(m+n)][2m-n-11(m+n)]
=(2m-n+11m+11n)(2m-n-11m-11n)
=(13m+10n)(-9m-12n)
=-3(13m+10n)(3m+4n)