Please provide the order of internal and leaf nodes. Considering order = 3 for both internal and leaf nodes, means max keys that can be present is 2 and min 1, I am wrinting this answer.
Step 1: Delete 15 from leaf node.
Step 2: Now that 15 is deleted from leaf node we have an underflow, first thing we check to fix underflow is if we can borrow minimum number of keys (1) from siblings to fix it. Here we can borrow a key from node containing (3,6,9). Borrow 9 and place it in the empty node.
Step 3: Now we need to fix the structure of B+ Tree, the internal node containing 15 is replaced with 9.
Done.
