Try to sketch a figure.
If ABCD is a parallelogram,
The coordinates of a vector, when given the coordinates of points is :
Then, if it's a cube, diagonals may be equal and perpendicular (i don't remember if this condition is necesary).
If 2 vectors are perpendicular, their scalar product is 0.
The scalar product is the sum of the product of each coordinate (x with x, y with y etc..)
Try to work with these elements