y=(x^2-3x+6)/x^2=1-3/x+6/x^2,
y|[x=3]=2/3,
y’|[x=3]=(3/x^2-12/x^3)|[x=3]=-1/9,
法线斜率k=9,
法线方程y-2/3=9(x-3),y=9x-79/3.