I have this question that I am really stuck with, I have done the first part and believe I am close to solving the second part but really cannot see what to do next.

Q) ABCD is a trapezium with BC parallel to AD.

AB=3b BC=3a AD=9a

M is the midpoint of BC and N is the midpoint of AD

a) Find the vector MN in terms of a and b - which i found to be 3a-3b

b) Prove that XY is parallel to AD

if anybody could help me with this part it would be much appreciated

Thanks