As you've said, $$ \sigma = My / I $$
At a particular location (i.e. 2m from A), M and I are constant. The question asks for maximum bending stress at that location, so you need maximum y.
So, you need to work out where the neutral axis is, and whether top face or bottom face is furthest from it, and use y for that face.
(Maximum bending stress is very likely to mean maximum magnitude of bending stress, so while one face will give you a positive y and the other a negative y, just use the one with the largest magnitude. Keep the sign in order to determine whether it is compressive or tensile stress.)