$R_3 \rightarrow R_3-a R_1\left[\begin{array}{ccc}
1 & a & 0 \\
0 & 1 & a \\
0 & -a^2 & 1
\end{array}\right]$
$\begin{aligned} & \begin{array}{l}R_1 \rightarrow R_1-a R_2 \\ R_3 \longrightarrow R_3+a^2 R_2\end{array}\left[\begin{array}{ccc}1 & 0 & -a^2 \\ 0 & 1 & a \\ 0 & 0 & 1+a^3\end{array}\right] \\ & 1+a^3=0 \Rightarrow a^3=-1 \text { i.e., } a=-1 \text {. } \\ & \end{aligned}$
If $a=-1$, the coefficient matrix has got rank 2 .
$\therefore$ The system has infinite number of solutions if $a=-1$.