3 x²-2x-4=0
3(x^2-2x/3)-4=0
3(x-1/3)^2-4-1/3=0
(x-1/3)^2=13/9
所以 x-1/3=√13/3
x=(1+√13)/3
或 x-1/3=-√13/3
x=(1-√13)/3