x²-3x=3x²-3x+9/4=3+9/4(x-3/2)²=21/4x-3/2=±√21/2x=(3-√21)/2，x=(3+√21)/2
x^2-3x+9=12(x-3)^=12x-3=2根号3或-2根号3x=3+2根号3或x=3-2根号3
x^2 - 3x =3x^2 - 3x+2.25=5.25(x-1.5)^2=5.25然后开方，你几年级？
