Unmet dependencies unable to fix

Your unmet dependencies problem is caused by the MySQL APT Repository repository. MySQL Server 5.6 from the MySQL APT Repository requires libstdc++6 >=4.9 however MySQL Server 5.6 (mysql-server-5.6) from the default Ubuntu repositories requires libstdc++6 >=4.6. When you install the version of MySQL Server 5.6 (mysql-server-5.6) from the default Ubuntu repositories, it automatically resolves all the package dependencies for you.

To fix the problem you need to remove the MySQL APT Repository and then uninstall all the packages that you installed from the MySQL APT Repository.

To remove the MySQL APT Repository, use a command of the form:

sudo dpkg --remove package-name

where you replace package-name with the package name of the MySQL APT Repository you previously installed. Or else you can also search in the Ubuntu Software Center for the MySQL APT Repository .deb package that you installed and remove it from there.

Then install the MySQL Server 5.6 with the command:

sudo apt install mysql-server-5.6