Are you talking about the 20 17 national volume 3 science mathematics question 20?
Judging from the topic setting, this question means y^2 = 2x.
This is a parabola with a regular X shape.
At the same time, the L line intersects the parabola at two points.
Obviously, the slope of this L line cannot be zero, but it may not exist.
For a straight line whose k can't be 0, but it may not exist.
Let x=my+b be the basic mass ... where m= 1/k, and when k does not exist, m=0.
Of course, you can also choose to set y=kx+b as usual, and then add that if K doesn't exist, it doesn't meet the meaning of the question ... take another step!