Answer
$(a). B^{2}$ is undefined.
$(b). F^{2}$ is defined ,view answer in the image below.
Work Step by Step
$(a)$. $B^{2}$ is undefined, because $B$ has dimensions $2×3$ so the rows won't match with the columns.
$(b). F^{2}$ is defined ,because $F$ has dimensions $3×3$ so the rows will match with the columns. View image below.