\[\begin{array}{l}
DK:\left\{ \begin{array}{l}
5 - x \ge 0\\
2x + 3a \ge 0
\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}
x \le 5\\
x \ge - \frac{{3a}}{2}
\end{array} \right.\\
De\,txd\,la\,mot\,doan\,co\,do\,dai\,bang\,2\,thi\\
\,5 - \left( { - \frac{{3a}}{2}} \right) = 2 \Leftrightarrow 5 + \frac{{3a}}{2} = 2 \Leftrightarrow 3 + \frac{{3a}}{2} = 0 \Leftrightarrow 1 + \frac{a}{2} = 0\\
\Leftrightarrow \frac{{2 + a}}{2} = 0 \Leftrightarrow a = - 2
\end{array}\]
