3x²-2x-7=0
3x²-2x=7
x²-2/3x=7/3
x²-2/3x+1/9=7/3+1/9
(x-1/3)²=22/9
x-1/3=±√22／3
x1=1/3+√22／3, x2=1/3-√22／3