Proving that diagonals of a parallelogram bisect each other

[frozen7]

I am not sure whether this question should be posted here or not.

4. Using vector methods, prove that the diagonals of a parallelogram bisect each other.

I have no idea at all how to start. Any clues?

 

[Tide]

HINT: Look for congruent triangles.

Also, this is a math problem - not a physics problem.