I do know what a polynomial is..... but this problem I don't understand

( y2-3y+6) (y1-3y2+y3) =

y3+y2-3y2-3y+y+6 =

Why does the above equal y3-2y2-2y+6? I know you have to combine the like terms, Like terms are like y2 - 3y2 Right???

How are they adding the like terms, these are the steps I'm not understanding.