I think answer will be 14
We have 5,6,9,10
Now if we take 10 at root then 5 trees will be there 4 with height 3
and 1 with height 2
If we take 10 then for height 3 -4 trees
Similarly for 5 if it is selected to insert at root node then 5 trees possible
Now if we insert 6 at root node then definitely 5 will be it's left child so 2 trees are possible by arranging 9 and 10 as it's right child
And same will be for 9 also two trees are possible with height 2
H(3)4+H(3)4+H(2)1+H(2)1+H(2)2+H(2)2