Extend e'd' and f'd' to a'b', which intersect at h' and g' respectively.

Make their extension lines intersect ed and fd at h and g respectively. There is ab in g''h''

It can be understood that the projections of line gh and line g''h'' on plane X are both g'h'.

G''h'' is on the ab line, while gh is on the surface def. Therefore, if the extension line hg and h "g" intersect at n, it can be considered that n is on both the line ab and the surface def, that is, the intersection of the line ab and the surface def.

Therefore, the position of k can be obtained. (The picture is drawn casually, you know)