we have,
vectors →a,→b,→c are satisfy conditions
→a×→b=→b×→c≠0
⇒→a×→b=−→c×→b
⇒→a×→b+→c×→b=→0
⇒(→a+→c)×→b=→0
Then, →a+→c are parallel
Now,
→b=k(→a+→c)
Where k is the scalar.