[krit]

$จะได้ (x+3)P(x)+(x+3)=-x^3+6x^2+21x-18$
$\ \ \ \ \ \ (x+3)(P(x)+1)=-x^3+6x^2+21x-18$
$\ \ \ \ \ \ P(x)+1=-x^2+9x-6$
$\ \ \ \ \ \ P(x)=-x^2+9x-7$
$\therefore ผลคูณของส.ป.ส. ทุกพจน์=-1\times9\times-7=63$