What should be added to x2+xy+y2 to obtain 2x2+3xy?
Let p should be added. Then according to question, x2+xy+y2+p=2x2+3xy
⇒p=2x2+3xy−(x2+xy+y2)
⇒p=2x2+3xy−x2−xy−y2
⇒p=2x2−x2−y2+3xy−xy
⇒p=x2−y2+2xy Hence, x2−y2+2xy should be added.