The number of nontrivial solutions of the system x –y + z = 0, x + 2y – z = 0, 2x+y+3z=0 is        

  $\left|\begin{array}{ccc}1& -1& 1\\ 1& 2& -1\\ 2& 1& 3\end{array}\right|=9\ne 0$

Given equations have trivial solution only.

$\therefore$ No of  non trivial solutions =0