Answer
$(a). DB+DC$ is defined, view answer in the image below.
$(b).BF+FE$ is undefined.
Work Step by Step
$(a). DB+DC=D(B+C)$ (distributive property).Start by adding matrix $B$ to matrix $C$ then multiply the sum by$D$. View image below.
$(b). BF+FE$, start by multiplying matrix $B$ by matrix $F$ and matrix $F$ by matrix $E$. The products have different dimensions so we can't add matrices of different dimensions. View image below.
