(x+y+1)(x-y+1)
=(x+1)^2-y^2
=x^2+2x+1-y^2
(a-2b+3c)^2
=(a-2b)^2+2(a-2b)3c+(3c)^2
=a^2-4ab+6ac-12bc+4b^2+9c^2