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?