Given $A=\left[\begin{array}{cc}2& 8\\ 3& 0\\ 5& 1\end{array}\right],B=\left[\begin{array}{cc}2& 0\\ 3& 8\end{array}\right],$ and $C=\left[\begin{array}{cc}7& 2\\ 6& 3\end{array}\right]$ : $($a$)$ Is $AB$ defined? Calculate $AB.$ Can you calculate $BA?$ Why? $($b$)$ Is $BC$ defined? Calculate $BC.$ Is $CB$ defined? If so$,$ calculate $CB.$ Is it true that $BC=CB?$