I installed ffmpeg-3 and ssr-0.3.11, both Slackbuilds packages, but as the codecs were not working, I decided to install the most current versions of the sources, from both programs. It didn't work anyway, so I uninstalled it. To make the mess worse, I installed an older ffmpeg later, 2. It didn't work. After that I reinstalled the versions of Slackbuilds, but now the simplescreenrecorder (ssr) does not open because it says that it did not find libavformat.56; but ssr needs libavformat.57, so there is something wrong there and I can't undo this mess. Is there a program for Slackware that looks for conflicts and tries to solve this kind of mess?

look into what you installed for them.
then just pick out what ones you do not want installed then use that information to remove them.
then take a moment to think about what ones you need to (re)install then try it again. Now you know how to get yourself out of that situation.
this is recompiled ffmpeg alienBobs
http://www.slackware.com/~alien/slac.../ffmpeg/pkg64/

just move up the parent directory if you are running 32bit slack to get that one.

2. or, you might have to hunt down libavformat* the look at its links, sometimes they just get soft link by the name it needs to be to have other software use it. ls -la /lib64/libavformat*

It is a simple solution that solved my problem! I really appreciate the answer!