Hello @linda6,
Almost everything about this post looks good! I will add several things.
The posting key is also used for voting (you may have known this, but you only said posting and commenting, so I wanted to emphasize that)
The memo you sent me did not require the memo key. In order to send an encrypted message, you must put a "#" in front of the message. Here's what the encrypted form of that message would have needed to look like:
"# Hello! cmp2020"
Here you can see my response, but you would need your memo key to see it in your wallet.
That being said, I think you have a pretty good understanding of key security! I ask you to move on to the next achievement: posting etiquette.