Tips och lösning till U 7.6k

CD=\left(\begin{array}{rrr}1&2&3\\4&5&6\\7&8&9\end{array}\right)_{3\times\underline{3}} \left(\begin{array}{r}3\\2\\1\end{array}\right) _{\underline{3}\times1} =\left(\begin{array}{r} 10\\ 28\\ 46\end{array}\right).