The question is determine the value of x for which the vector c = xi + j + k is coplanar with the vectors a ( i +j -k) and B (2i - j + 3k).

The anser is x = 4 and i think u need to use vector triple cross product to find it but i cant seem to manage it or find any examples of this style of question. Its seems to be a common style of question on most of the past papers of which my exam is tomoz.

Thanks in advance John