已知方程组 $\left( \begin{array}{rrr}1 & 2 & 1\\ 2 & 3 & a + 2\\ 1 & a & -2 \end{array} \right)\left( \begin{array}{l}x_{1}\\ x_{2}\\ x_{3} \end{array} \right) = \left( \begin{array}{l}1\\ 3\\ 0 \end{array} \right)$ 无解, 则 $a =$