Right choice is (a) (AB)’=\(\begin{bmatrix}-6&-21&-24\\8&28&32\\2&7&8\end{bmatrix}\)
The best I can explain: Given that, A=\(\begin{bmatrix}2\\7\\8\end{bmatrix}\), B=\(\begin{bmatrix}-3&4&1\end{bmatrix}\)
AB=\(\begin{bmatrix}2\\7\\8\end{bmatrix}\)\(\begin{bmatrix}-3&4&1\end{bmatrix}\)=\(\begin{bmatrix}-6&8&2\\-21&28&7\\-24&32&8\end{bmatrix}\)
∴To find (AB)’, interchange the rows with columns and columns with rows of the matrix AB
(AB)’=\(\begin{bmatrix}-6&-21&-24\\8&28&32\\2&7&8\end{bmatrix}\)