9x^2+y^2+2z^2-18x+4z-6y+20=0

<=>(9x^2-18x+9)+(y^2-6y+9)+(2z^2+4z+2)=0

<=>9(x^2-2x+1)+(y-3)^2+2(z^2+2z+1)=0

<=>9(x-1)^2+(y-3)^2+2(z+1)^2=0

<=>x=1;y=3;z=-1

(x ; y ; z) = (1 ; 3 ; -1)