解：??$(1)$??將點(diǎn)??$B(2，$????$-4)$??代入反比例函數(shù)得，??$-4=\frac m 2，$????$m=-8$??
??$ ∴y=-\frac 8x$??
令??$x=-4，$????$n=-\frac 8{-4}=2$??
將點(diǎn)??$A(-4，$????$2)、$????$B(2，$????$-4)$??代入一次函數(shù)表達(dá)式得??$\begin{cases}-4k+b=2\\2k+b=-4\end{cases} $??
??$ ∴y=-x-2$??
??$ (2)$??令??$y=0，$????$-x-2=0，$????$x=-2$??
??$ ∴C(-2，$????$0)$??
??$ S_{△AOB}=\frac 12×6×6-\frac 12×6×2-\frac 12×6×2=6$??
??$ (3)$??由圖可知，??$-4＜x＜0$??或??$x＞2$??
