2x^2+6x+2*9/4=9+2*(9/4)
2(x+3/2)^2=27/2
(x+3/2)^2=27/4
x+3/2=正负2分之3倍根号3
x=负2分之3倍根号3-3/2 或者2分之3倍根号3-3/2