设圆为：x^2+y^2+dx+ey+f=0
代入A：1+4+d+2e+f=0,得：d+2e+f=-5
代入B：9+16+3d+4e+f=0,得：3d+4e+f=-25
消去e得：f=d+15
X轴上弦长：y=0时,得：x^2+dx+f=0
|x1-x2|^2=6^2=36=(x1+x2)^2-4x1x2=d^2-4f,得：d^2-4f=36
代入f得：d^2-4d-96=0
(d-12)(d+8)=0
得：d=12 or -8
d=12时,f=d+15=27,e=-(5+d+f)/2 =-22,方程为：x^2+y^2+12x-22y+27=0
d=-8时,f=d+15=7,e=-(5+d+f)/2=-2 ,方程为：x^2+y^2-8x-2y+7=0