What is the chord in figured bass when the bass note is B flat with 7 underneath, in the key of D flat major?
The note before is A flat with 6 underneath (iiib???) and the oneafter is F with 6 underneath (Ib).
You have given only minimal information. Let us analyze the possibilities. You state that the piece is in D-flat Major. You give the bass note with the figuring provided. In the first sonority, A-flat is the bass and 6 is given as the figure. Since 6 denotes fist inversion, we must accept that this is first inversion f minor, a iii6 chord in D-flat Major. In the second sonority, B-flat is the bass and 7 is given as the figure. This denotes a 7 chord in root position. In D-flat Major, a seven chord built on B-flat is B-flat minor seven, or B-flat, D-flat, f, and A-flat. In D-flat Major this is a vi7 chord. In the third sonority you provide, F is the bass and 6 is given as the figure. Again, this denotes first inversion, therefore the sonority must be D-flat Major in first inversion. This progression is not very strong movement: iii6, vi7, I6, however there is no reason why it could not occur.