I need to show that the diagonals of a parallelepiped bisect each other. ie 2 lines going from opposite corner to opposite corner.

ive tried experimenting with letting a b and c be vectors going in the 3 possible directions but i havnt got far. ive also tried to solve the problem by assuming the point M is in fact the bisect point and going from there but i keep hitting dead ends

any help would be much appreciated