Answer
$(a). ABE$ is defined, view answer in the image below.
$(b). AHE$ is undefined.
Work Step by Step
$(a). ABE$ can be performed, view imagine below.
$(b)$. $AHE$ is undefined because, $AH$ has dimensions $2×2$ and $E$ has dimensions $3×1$ so the rows won't match the columns. View image below.