the repo does not exist in mint. try to copy the url and remove the "?channel=trusty-partner" attribute of this url. or just open your package manager and search for "adobe-flashplugin" you will see it is already installed..but not up2date ..
install for chromium browser:
sudo apt-get install pepperflashplugin-nonfree
sudo add-apt-repository ppa:nilarimogard/webupd8
sudo apt-get update && sudo apt-get install freshplayerplugin pepperflashplugin-nonfree
edit: validate your installed flash version .. open "about:addons" as url in your browser and search for flash.
...... on my fresh mint 17.2 is "Shockwave Flash 11.2 r202.481" installed.