Your question doesn't make much sense. But for any scale, "do" would be the first note of the scale, which is whatever the scale is called. But for a flat major, from what it sounds like you're asking, the note would be a flat. Ex: In B flat major, "Do" is b flat.
A-flat Major scale
The subdominant in a scale is the 4th, which in the key of B-flat major is E-flat.
C flat Major
e-flat major
The Keys of B-flat, E-Flat, A-Flat, D-Flat, G-Flat, C-Flat, and F-Flat major all contain the note E-flat. F-flat major is a key which only exists in theory and not in practice, since there is a double flat in that scale (subdominant). The major scale with the most flats is C-flat major - with all seven flats.
if it's an A-Flat scale the first note is A-Flat...
Eb Major
F major
F major
Ab Major
The E flat scale
None. The F major scale has a B flat.