a^2-4b^2+3a-6b
=(a-2b)(a+2b)+3(a-2b)
=(a-2b)(a+2b+3)
4m^2-4m+2n-n^2
=4m^2-n^2-(4m-2n)
=(2m-n)(2m+b)-2(2m-n)
=(2m-n)(2m+b-2)