$ sudo apt-get update
$ sudo apt-get install postgresql-9.3-postgis-2.1 

postgresql-9.3-postgis-2.1 is installed in your system.

Make ensure the postgresql-9.3-postgis-2.1 package were installed using the commands given below,

$ sudo dpkg-query -l | grep postgresql-9.3-postgis-2.1 *

You will get with postgresql-9.3-postgis-2.1 package name, version, architecture and description in a table

postgresql-9.3-postgis-2.1 package basic information: