Question

Let a,b,c be three non zero vectors which are pairwise non collinear

Answer 1

let a+2 b = u c  ............1


and   b+3c =va ............2


put replace vctor a from 1 and b


 b+3c =v(uc-2b)


or  (1+2v)b=(uv-3)c


since b and c are not colinier so


 1+2v =0   or v=-1/2


and uv-3=0   or u=-6


put value of u in equation 1


a+2b=-6c


a+2b+6c=0
i hope this will help you