$$\left. \begin{array} { l } { A = \{ \begin{array} { l } { 5 , 7 , 8 \} } \\ { = \{ 3 } \\ { A U _ { B } = } \end{array} } \\ { = } \end{array} \right.$$
$A=l$
$l\in \mathrm{C}$
$A=l$
$l\in \mathrm{R}$
