x^2+xy-yz-xz=x(x+y)-z(x+y)=(x-z)(x+y)4a^2+3c-3ac-4a=4a(a-1)-3c(a-1)=(4a-3c)(a-1) -4a^2-12a(x-y)-9(x-y)^2 =-[3(x-y)+2a]^2ab-b^2-bc+ac=b(a-b)+c(a-b)=(b+c)(a-b) 4/9m^2-4/3(a+b)m+(a+b)^2=[(a+b)-2/3m]^2