Decompose the vector into vectors which are parallel and perpendicular to the vector

let and

let where u is vector parallel to b and v is vector perpendicular to b

So, ; where p is some constant

Substituting this value in above equation

Now according to conditions since vector v and b are perpendicular to each other

6 – p – 3 – p – 6 – p = 0

P = –1

So,

Substituting this value of in above equation