x^2-9=(x^2-6x+9)/2
2(x^2-9)=(x^2-6x+9)
2(x-3)(x+3)-(x^2-6x+9)=0
2(x-3)(x+3)-(x-3)^2=0
(x-3)[2(x+3)-(x-3)]=0
(x-3)(2x+6-x+3)=0
(x-3)(x+9)=0
x=-3或x=-9