I mistakenly mis-configured Synaptic Package Manager - how do I reset
 

I was trying to add jessie-backports repository to the existing Synaptic Package manager on my Mint v18 (which I know I did in error) and even though it gave me some error messages, it apparently submitted. When I reran the Synaptic Software Manager, it now gives me the following error message:

E: Malformed entry 2 in list file /etc/apt/sources.list.d/additional-repositories.list (Suite)
E: The list of sources could not be read.
Go to the repository dialog to correct the problem.
E: _cache->open() failed, please report.

Also now when I start the Mint "Software Manager" after authenticating, it "goes away" without any error message or description. I think this condition and the Synaptic Package Manager error are related.

I do I reset my Package Manager to what I had before I made this mistake?

As root, edit the /etc/apt/sources.list file and remove or remark out the problem entry, then test.

frankbell's advice is fine - except that the file to edit is /etc/apt/sources.list.d/additional-repositories.list (as noted in the error message).
But one thing to try is to open the Software Sources program.
Menu > Administration > Software Sources
and click the button at the bottom that says "Restore the default settings"
I am not 100% positive that will fix the additional-repositories.list
It may just reset the /etc/apt/sources.list.d/official-package-repositories.list
But you can still use the Software Sources program to delete the offending entries. Click on "Additional repositories" and uncheck the ones you don't want. Then click the "Update the cache" button.

that "Restore the default settings" is the best option,
& will reset the list back to it's defaults,
which should fix the additional-repositories.list. .. ..

it's also probably the preferred method, as well.
- rather than editing /etc/apt/sources.list.d - - that is.