y=x+b
y^2=-4x
(x+b)^2=-4x
x^2+(2b+4)x+b^2=0
(2b+4)^2-4b^2>0
16b+16>0
b>-1
x1+x2=-2（b+2),x1x2=b^2
M(x0,y0)A(x1,x1+B),b(x2,x2+b)
k1=(x1+b-y0)/(x1-x0),k2=(x2+b-y0)/(x2-x0)
k1+k2=((x1+b-y0)(x2-x0)+(x2+b-y0)(x1-x0))/(x1-x0)(x2-x0)
=(x1x2-x0x1+bx2-bx0-y0x2+x0y0+x1x2-x0x2+bx1-bx0-y0x1+x0y0)/(x1-x0)(x2-x0)
=(2x1x2+2x0y0-x0(x1+x2)+b(x1+x2)-2bx0-y0(x1+x2))/(x1-x0)(x2-x0)
=(2b^2+2x0y0+(b-x0-y0)*(-2(b+2))-2bx0)/(x1x2-x0(x1+x2)+x0^2)
=(2b^2+2x0y0+(b-(x0+y0))*（-2（b+2)-2bx0)/(b^2-x0(-2(b+2)+x0^2)
=k
解出x0,y0
x0=4+-2*6^1/2,y0=2