print (L[2:6]) means print all the values in list from index value 2 to 6
Since, index starts from 0, so the value at index 2 is 4 and at index 6 is 20.
But the item at position 6 will not be included.
Therefore output will be [4,9,10,11].
Note: The search will start at index 2 (included) and end at index 6 (not included).
Remember that the first item has index 0.