Final answer:
A Perfect Fifth below F-flat is B-double flat, which is seven semitones below F-flat.
Step-by-step explanation:
A Perfect Fifth below F-flat would be B-double flat (Bbb). This is because in musical interval terms, a perfect fifth consists of seven semitones. Starting from F-flat and counting down seven semitones, you would land on B-double flat (Fb-Eb-Db-Cb-Bb-Abb-Bbb).