Hi Cao,

The syntax of list slicing is 
seq = my_list [start: stop: step] 

So in this case, when you are trying with  a = [0,1,2,3,4,5,6,7,8,9] and slicing as a[:-6:-1] it reads the elements in reverse direction as step is -1 and cretes the list from starting index to the index -6(excluding index -6).So the output(new list created) is 
>>> a[:-6:-1]
[9, 8, 7, 6, 5]

Similarly, in [:-6:-2], the step is -2 and hence the interpreter will read each alternative element from the last index.So the result is 
>>> a[:-6:-2]
[9, 7, 5]

Hi Cao,

Interesting question, as we discussed in the session 

column name[first index: last index:sorting]

when we want to sort in descending order we use the (second : ).

Hope it helps