I think its simple...i think i just have a brain block lol...mayb cuz its 12 at night.

OABC is a rectangle.

Answer x = 6.A has position vector (2, -3, 5) (with respect to origin O)

B has position vector (x, 3, 7)(with respect to origin O)

So i thought, these must be perpendicular! Scale product...well...i got s = 13 which is wrong .

But also if these are perpendicular...won't they share the same x-coordinate? Or Y co-ordinate?

Cheers in advanced for any help you can give me .