1)ab(a^2-4b^2)=ab(a-2b)(a+2b)
2)7(a^2-2ab+b^2)=7(a-b)^2
3)(x^2+2x+1)-(y^2+6y+9)=(x+1)^2-(y+3)^2=(x+1-y-3)(x+1+y+3)=(x-y-2)(x+y+4)
