(x²-nx+3)(3x+2)
=3x³ +2x²-3nx²-2nx+9x+6
=3x³ + (2-3n)x² - 2nx + 9x + 6
不含 x² 项,意思是说 x² 项的系数 2-3n =0,所以,n=2/3.