Successor of 6 is 7 or 9?
There are many ways to traverse a tree, and what is the 'successor' of 6 depends entirely on how you are traversing the tree. To illustrate this, here's the answer for some common traversal methods:

Pre-order traversal: 3.
In-order traversal: 9.
Post-order traversal: 17.
Level-order left-to-right: 18.
Lever-order right-to-left: 20.  

And for completeness, you can get 7 as a successor of 6 through reverse pre-order.