(2a-b)(a+b-c)-(a+2b)(a-b+c)+3c(a+b)
2a²+2ab-2ac-ab-b²+bc-(a²-ab+ac+2ab-2b²+2bc)+3ac+3bc
2a²+2ab-2ac-ab-b²+bc-a²+ab-ac-2ab+2b²-2bc+3ac+3bc
a²+b²+2bc
a²+2bc+b²
(2a-b)(a+b-c)-(a+2b)(a-b+c)+3c(a+b)
2a²+2ab-2ac-ab-b²+bc-(a²-ab+ac+2ab-2b²+2bc)+3ac+3bc
2a²+2ab-2ac-ab-b²+bc-a²+ab-ac-2ab+2b²-2bc+3ac+3bc
a²+b²+2bc
a²+2bc+b²