First, the six data records can be divided among the B-tree leaves in the following ways: 2-2-2 or 3-3. There are no other ways to divide size records, with 2 or 3 for each leaf. bcoz of it should be half full , so we cant divide like 1-5 or 1-1-2-2 , there are only 2 possible way .
In each of these two cases, there can be only one other node, the root, and it must point to each of the two or three leaves. Again, there are no other ways to structure 2 or 3 leaves, with each interior node having 2 or 3 children. Our conclusion is that there are only two different B-trees for 6 data records.
http://infolab.stanford.edu/~ullman/dbsisols/sol4.html#sol43