9(2m+n)^2-4(m-2n)^2
=[3(2m+n)]^2-[2(m-2n)]^2
=[3(2m+n)+2(m-2n)][3(2m+n)-2(m-2n)]
=(6m+3n+2m-4n)(6m+3n-2m+4n)
=(8m-n)(7n-4m)