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 can't seem to manage it or find any examples of this style of question.

Thanks in advance John

problem solved the cross product of axb doted with c = o as they are perpendicular so can solve from that to give anser