x^4 + 9x² + 4 - 6x³ + 4x² - 12x - 3x² + 9x - 6 + 2 - x = 0
x^4 - 6x³ + 10x² - 4x = 0
x(x³-6x²+10x-4)=0
x1 = 0
x³ - 6x² + 10x - 4 = 0
(x³ - 2x²) - (4x² - 8x )+ (2x - 4 )= 0
x²(x-2)-4x(x-2)+2(x-2)=0
(x - 2)(x² - 4x + 2) = 0
x2 = 2
x² - 4x + 2 = 0
D=16-8=8
x3 = (4-2√2)/2=2 - √2
x4 = (4+2√2)/2=2 + √2
x1+x2+x3+x4=0+2+2-√2+2+√2=6