3x²-(3a+3)x+3a+(a²-7)x=0
3x²+(a²-3a-10)x+3a=0
相反数x1+x2=0
所以-(a²-3a-10)=0
(a-5)(a+2)=0
则3x²+3a=0
x²=-a>0
所以a