Where lambda is 2 you've found the point of intersection between the line and the plane.
This point isn't reflected in the plane, because it lies on it, so you know this point lies on line 2.
You know another point on line 1, (2,4,-6), and to reflect any point in a line or plane, you go along perpendicular from that point to the line, and go the same distance on the other side.
So if we went perpendicular from the point (2,4,-6) to the plane, we've just gone across the direction of the normal to the plane (1,-2,1).
So now we have the equation of the line from point (2,4,-6) to the plane.
We can now work out the point of intersection between this line and this plane. And then from there you go along the same distance, to get your second point on line 2.