(a-b-c)(-b-a+c)
=(a-b-c)(-(a+b-c))
=-(a-c-b)(a-c+b)
=-((a-c)-b)((a-c)+b)
=-((a-c)^2-b^2)
=-(a^2-2ac+c^2-b^2)
=b^2-a^2-c^2+2ac