Hello,

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