10 / \ / \ 5 15 / / \ / / \ 2 12 20 / / \ / / \ 11 18 21 \ \ 25Walking back up the path to the newly inserted node, all of the nodes are height-balanced except the root, holding 10. Two steps downward from there toward the higher subtree takes two steps to the right, so this is a zig-zig, calling for a single rotation. After doing that single rotation, the tree is as follows.
15 / \ / \ / \ 10 20 / \ / \ / \ / \ 5 12 18 21 / / \ / / \ 2 11 25