解：?$(2)$?因?yàn)?$(a，3)$?為?$“$?美好組?$”，$?則?
$a + 3=a×3，$?
?$ $?解得?$a=\frac {3}{2}。$?
?$ (3)$?解方程組?$\begin {cases}x - 2y=m - 9\\2x + y=2m - 7\end {cases}，$?
可得：?$\begin {cases}{x=m - 4.6}\\{y=2.2}\end {cases}$?
?$ $?若?$(x，y)$?是?$“$?美好組?$”，$?則
?$m - 4.6+2.2=(m - 4.6)×2.2，$?
?$ $?解得?$m=\frac {193}{30}。$?
?$ $?所以當(dāng)?$m=\frac {193}{30}$?時，?$(x，y)$?是?$“$?美好組?$”。$?
