The blue box is your plane. You can create the plane variable by using 3 points of that box.
Code:
new Float:plane[4];
xs_plane_3p(plane, box_point1, box_point2, box_point3);
Then you need the start position of the line and the direction it goes.
If all you have is 2 points, then you need to calculate the direction.
Code:
new Float:line_direction[3];
xs_vec_sub(line_point2, line_point1, line_direction);
Finally, apply variables to check if the line intersects the plane.
Code:
new Float:intersection[3];
if( xs_plane_rayintersect(plane, line_point1, line_direction, intersection) )
{
// Line intersects the plane at "intersection" point
}
__________________