(d) The given system of homogeneous equations has a non-zero solution if,
$\Delta = 0$
i.e., $\left| {\,\begin{array}{*{20}{c}}1&{\,\,\,1}&{ - 1}\\3&{ - \alpha }&{ - 3}\\1&{ - 3}&{\,\,\,1}\end{array}\,} \right| = - 2\alpha - 6 = 0$ ,i.e. if $\alpha = - 3$.