设直线L y=2x+b
代入y^2=-4x
4x^2+4bx+b^2=-4x
4x^2+(4b+4)x+b^2=0
由根与系数的关系
x1+x2=-b-1
x1*x2=b^2/4
|x1-x2|^2=(x1+x2)^2-4x1x2=(-b-1)^2-b^2=2b+1
|AB|=√15
|AB|^2=15
(x2-x1)^2+(y2-y1)^2=15
[y2-y1=(2x2+m)-(2x1+m)=2(x2-x1)]
5(x2-x1)^2=15
2b+1=3
b=1
直线L y=2x+1