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# deb cdrom:[Official Debian GNU/Linux Live 9.1.0 lxde 2017-07-23T01:38]/ stretch main
#deb cdrom:[Official Debian GNU/Linux Live 9.1.0 lxde 2017-07-23T01:38]/ stretch main
deb http://ftp.us.debian.org/debian/ stretch main contrib non-free
deb-src http://ftp.us.debian.org/debian/ stretch main contrib non-free
deb http://security.debian.org/debian-security stretch/updates main contrib non-free
deb-src http://security.debian.org/debian-security stretch/updates main contrib non-free
# stretch-updates, previously known as 'volatile'
deb http://ftp.us.debian.org/debian/ stretch-updates main
deb-src http://ftp.us.debian.org/debian/ stretch-updates main
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Yes, that's known that Debian removed it in Stretch, because it's been broken for quite a while. Search these forums for Flash--there are quite a few ways to get it installed, ranging from just downloading and installing a deb package to the more involved manual download and install of the .so file.