How to delete an element from a B-tree
All of the keys you want to delete in this particular example belong to the case when we are deleting keys from a leaf node, without need to rebalance a tree after deletion.

Algorithm for deleting a key from a leaf node consists of three steps:

1) Search for the value to delete.

2) If the value is in a leaf node, simply delete it from the node, because there are no subtrees to worry about.

3) If underflow happens (i.e., if we break one of the defining rules/properties of a B-tree - every non-root node has to have at least (M-1)/2 keys), rebalance the tree.

Also, if you are interested in implementation of rebalancing algorithm, beyond the scope of the given example, I suggest these sources:

https://www.youtube.com/watch?v=vqIxJaNrPI4

https://webdocs.cs.ualberta.ca/~holte/T26/del-b-tree.html