设M点的坐标为（x,y)
|MA|=|MB|
[(-3-x)^2+(2-y)^2]^(1/2)=[(3-x)^2+(-2-y)^2]^(1/2)
[(-3-x)^2+(2-y)^2]=[(3-x)^2+(-2-y)^2]
9+6x+x^2+4-4y+y^2=9-6x+x^2+4+4y+y^2
12x-8y=0
3x-2y=0
即证.