Question

Answer using vector.......

Answer 1

both vector are perpendicular so A.B=0
(i+cj+5k).(2i+j-k)=0 i.e. 2+c-5=0 that mean c-3=0 so c=3

Answer 2

hey here is your answer.............
if two vectors r perpendicular then A.B=0
i+cj+5k . 2i+j-k =0
2+c-5=0
c-3=0
c=3