4(m+n-3)^2-1/4(4m-2n)^2=(2(m+n-3))^2-(1/2(4m-2n))^2=(2(m+n-3)+1/2(4m-2n))(2(m+n-3)-1/2(4m-2n))=(2m+2n-6+2m-n)(2m+2n-6-2m+n)=(4m+n-6)(3n-6)
