This Bb note is written in the space above the bass staff.
In Lesson 9 you learned that a flat sign written on a line at the beginning of the treble staff applied to all notes on that line. The same rule applies to a flat sign written on the bass staff. Also, the flat sign on the second line of the bass staff applies to all B notes above the staff. Thus, the 2nd note in bars 2, 3 and 6 is Bb in each case.