a-b=3
b-c=2
a-c=5→c-a=-5
a²-bc+b²-ac+c²-ab
=(a²-ab)+(b²-bc)+(c²-ca)
=a(a-b)+b(b-c)+c(c-a)
=3a+2b-5c
=3a+2(a-3)-5(a-5)
=3a+2a-6-5a+25
=19