(a+2b-c)(a-2b-c)-(a+b-c)^2
=(a-c)^2-4b^2-(a+b-c)^2
=[(a-c)+(a-c+b)][(a-c)-(a-c+b)]-4b^2
=2b(a-c)-4b^2
=2b(a-c-2b)