by saying that "it is defined by the Earth line", it is indicating that it is a plane that passes through the Earth line. The two traces of the drawing are in this line.
to understand this type of plane is usually very useful to see your profile.
We take the point given to the profile and join it with the ground line, giving us the profile of this plane.
We apply the general procedure of intersection of planes, at the point of cut of the trace Alpha one with the trace beta one, we obtain the trace H one, of the line of intersection, and the point of cut of the vertical traces of the planes generates the trace v two.
These two traces match. Therefore, we need a second common point to the two planes, to find their line of intersection.
We draw a straight from the alpha plane, parallel to the ground line. By point A, this line is plotted.
We perform the intersection of the straight R, with the beta plane.
for this, along this line we draw a horizontal plane.
We found the intersection line of these two planes.
At the intersection point of this line s, with the line r, we will obtain a point b, common to the three planes.
Joining this point with the point of the traces v two and h one, we draw the line of intersection sought.
Procedure in the dihedral system.