So since you are checking for an axis, and you already know the position of the ball and the angle (which you know from the velocity) then it is just a simple matter of trigonometry. Here, I made this function for you.
func get_intersect(pos, ang, y):
var height = y - pos.y
if ang * height < 0.0:
return null
var tan_ang = tan(ang)
if tan_ang == 0.0:
return null
var width = height / tan_ang
return Vector2(pos.x + width, y)
Position is the object position that is moving, the angle is in radians on the velocity (you can use velocity.angle() if you have a Vector2) and the y is the y position of the line you are interesting with. It returns the point of intersection (or null if nothing found). And also here is the sample project I made you can test out. Cheers.
