a
(a) The system will have a non-zero solution, if
$\Delta \equiv \left| {\,\begin{array}{*{20}{c}}{{a^3}}&{{{(a + 1)}^3}}&{{{(a + 2)}^3}}\\a&{a + 1}&{a + 2}\\1&1&1\end{array}\,} \right| = 0$
$ \Rightarrow \left| {\,\begin{array}{*{20}{c}}{{a^3}}&{3{a^2} + 3a + 1}&{3{{(a + 1)}^2} + 3(a + 1) + 1}\\{{a^2}}&1&1\\1&0&0\end{array}\,} \right| = 0$
by $\begin{array}{l}{C_2} \to {C_2} - {C_1}\\{C_3} \to {C_3} - {C_2}\end{array}$
==> $3{a^2} + 3a + 1 - \{ 3{(a + 1)^2} + 3(a + 1) + 1\} $
(expanding along ${R_3}$)
==> $ - 6(a + 1) = 0 \Rightarrow a = - 1$.