Let the vectors A = 3i - j + 5k and B = i +2j -3k , determine a vector C knowing that it is perpendicular to the OZ axis and that it verifies the relations

C dot A = 9 and C dot B = 4 .

if C ⊥ OZ then its z component is zero. So C = mi+nj

C•A = 3m-n = 9
C•B = m+2n = 4
solve that to get m and n, and you have
C = 22/7 i + 3/7 j