For problem 3 in section 2.2, you can decide whether a, b and c are row vectors or column vectors. Just be sure to be clear about what they are in each part.
For example, if in (b) you say bB is well defined and in (e) you say Bb is defined, then make sure to explain the dimension of b in each case.
Alternatively, you may give the dimensions of a, b, and c in the beginning of the problem and then keep them consistent in all parts.
“My friend asked me if I wanted a frozen banana. I said ‘No, but I want a regular banana later, so … yeah’.”