Write a vector equation connecting a, b, c and d to express the fact that vectorsAB=DC.

Deduce from your equation that

a)DA=CB,

b) if E is a point such that OAEC is a parallelogram, then OBED is a parallelogram.

For a)AB=b-aDC=c-d, so b-a=c-d. This can be rearranged to show that a-d=b-c , which shows thatDA=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 thatEBandODwere equal andEDandOBequal but without success.