So, answer for b) is $88$.
In order to put the minimum number of nodes in an AVL tree of height h, we must:
Because the maximum difference in height is 1 (one), the minimum sub trees are as follows:
Both sub trees have the minimum number of nodes
Therefore:
Relationship between n(h), n(h−1) and n(h−2): n(h) = 1 + n(h−1) + n(h−2) n(1) = 1 n(2) = 2
http://www.mathcs.emory.edu/~cheung/Courses/323/Syllabus/Trees/AVL-height.html