Minimum number of letters
n>1 means that minimum n value is 2.

(bb)^n means bb repeated n times, i.e. if n=2, then it's bbbb.

With n=2, it's "ba bbbbbb a" (I added spaces only for clarity).

With n=3, it's "ba bbbbbbbb aa" - as you see, teacher used n to regulate size of the first and the second repetition in sync.