A, B, D three-point * * line, then AB=mBD, that is: 2e 1+ke2=m(e 1-4e2).

That is, (2-m)e 1+(k+4m)e2=0, because e 1 and e2 are not * * * lines.

So: 2-m=0, k+4m=0, that is: m=2, k=-4m=-8.