（-x/3)^2+2x-1=0.
x^2/9+2x-1=0.
x^2+18x-9=0.
(x+9)^2-81-9=0.
(x+9)^2=90.
X+9=±3√10.
x=-9±3√10.
x1=-9+3√10.
x2=-9-3√10.