We have, AB= Position vector of B− Position vector of A=−6i^−2j^​+3k^∣AB∣=36+4+9​=7BC= Position vector of C− Position vector of B=−2i^+3j^​−6k^∣BC∣=4+9+36​=7CD=6i^+2j^​+k^∣CD∣=36+4+1​=41​DA=2i^−3j^​+2k^∣DA∣=4+9+4​=17​AC=−8i^+j^​−3k^=AB+BC∴ABCD is a quadrilateral which is not a parallelogram.