(a-b-2c)^2
=[(a-b)^2-2*2c(a-b)+4c^2]
=a^2+b^2-2ab-4c(a-b)+4c^2
=a^2+b^2-2ab-4ac+4bc+4c^2
=a^2+b^2+4c^4-2ab+4bc-4ac