Hello, usagi_killer!
The parabola and line intersect: .
. . Quadratic Formula: .
With two intersections, the discriminant must be positive:
. .
Hence: .
Make a sketch.
We have an upopening parabola with vertex (0,1)
. . and a straight line through the origin. Code:
*  *

*  *
*  *
*  * o
1* o
 o
 o
 o
     +       
o 
o 

If the slope is +2 or 2, the line is tangent to the parabola.
. . There is one intersection.
If is greater than 2 and less than 2, the line misses the parabola.
. . There is no intersection.
If is less than 2 or greater than 2, there are two intersections.