# discovering the package (ex.: dropbox)
$ grep "Package: dropbox" /var/lib/apt/lists/*
/var/lib/apt/lists/extra.linuxmint.com_dists_rosa_main_binary-amd64_Packages:Package: dropbox
/var/lib/apt/lists/extra.linuxmint.com_dists_rosa_main_binary-i386_Packages:Package: dropbox

# find the corresponding repository by searching the domain name
$ grep -r extra.linuxmint.com /etc/apt/*
/etc/apt/sources.list.d/official-package-repositories.list:deb http://extra.linuxmint.com rosa main #id:linuxmint_extra

# add apt repository
$ sudo add-apt-repository "deb http://extra.linuxmint.com rosa main"

# get the key from the warning
$ sudo apt-get update
W: GPG error: http://extra.linuxmint.com rosa Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3EE67F3D0FF405B2

# add the key
$ sudo apt-key adv --recv-key --keyserver keyserver.ubuntu.com 3EE67F3D0FF405B2

# update
$ sudo apt-get update

# install the package
$ sudo apt-get install dropbox