Answer
(a) $AB$ not defined.
(b)\begin{align*} BA&=\left[ \begin {array}{ccccc} 37&78&51&-104&124\\ 16& 26&28&-42&56\end {array} \right] . \end{align*}
Work Step by Step
Given $$ A=\left[\begin{array}{ccccc}{1} & {0} & {3} & {-2} & {4} \\ {6} & {13} & {8} & {-17} & {20}\end{array}\right], \quad B=\left[\begin{array}{ll}{1} & {6} \\ {4} & {2}\end{array}\right]. $$ We have
(a) $AB$ not defined.
(b)\begin{align*} BA&=\left[\begin{array}{ll}{1} & {6} \\ {4} & {2}\end{array}\right]\left[\begin{array}{ccccc}{1} & {0} & {3} & {-2} & {4} \\ {6} & {13} & {8} & {-17} & {20}\end{array}\right] \\ &=\left[ \begin {array}{ccccc} 37&78&51&-104&124\\ 16& 26&28&-42&56\end {array} \right] . \end{align*}
