x2+2xy+3y2-8y+6=0
而,x^2+2xy+3y^2-8y+6=(x+y)^2+2y^2-8y+6
=(x+y)^2+2(y-2)^2-2=0
（x+y)^2+2(y-2)^2=2
所以,|y-2|