Finding triangle vertices from midpoints as complex numbers

[MattL]

OK, I've got this question to do:

Find complex numbers representing the vertices of a triangle ABC given
that the midpoints of the sides BC, CA, AB are represented by complex numbers
z_1, z_2, z_3 respectively.

Thing is, I don't know where I'm taking the origin to be; if I took it at A then I would just have A is O, B is 2*z_3, C is 2*z_2. Surely there's more to the question than that?

 

[MattL]

yeah, oops, just realized it's about similar triangle and it doesn't matter where the origin is...