$$\left. \begin{array} { l } { a = ( - 7 ) , b = 9 c = ( - 11 ) , } \\ { \begin{array} { l } { a \times b = b \times a } & { } \end{array} } \end{array} \right.$$
$a=-7$
$b=9c$
$c\in \mathrm{R}$
