root@xubuntu:/# apt-get install linux-image-3.19.0-9-generic
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package linux-image-3.19.0-9-generic is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'linux-image-3.19.0-9-generic' has no installation candidate