(x²+ax+b)(3x-2)
=3x³-2x²+3ax²-2ax+3bx-2b
=3x²+(3a-2)x²+(3b-2a)x-2b
x²项的系数为7,3a-2=7,a=3
且积中不含x项,3b-2a=0,3b-2*3=0,b=2;