I now understand. No need to reply.
Write a vector equation connecting a, b, c and d to express the fact that vectors AB = DC.
Deduce from your equation that
b) if E is a point such that OAEC is a parallelogram, then OBED is a parallelogram.
For a) AB=b-a DC=c-d, so b-a=c-d. This can be rearranged to show that a-d=b-c , which shows that DA=CB
But for b) what should be done?? How can I deduce from the equation?? I tried finding an expression that included O,A,B,C,D and E so I could rearrange to show that EB and OD were equal and ED and OB equal but without success.