have you gone through NetBackup SQL Admin Guide? It is documented there.
What is your SQL Server version? NetBackup 7.6 is old and behind EOL, maybe you don't have a supported combination.
Do you perhaps mean clustered?
Regular failover cluster or AlwaysOn Availability Groups?
Are you aware of the fact that support for NBU 7.6 ended some years ago?
I am not sure that any compatibility lists for 7.6 are still available on Veritas web site - I have attached the last DB SCL that I have (dated June 2016).
Current compatibility lists can be found here: http://www.veritas.com/docs/000033647
To get back to your question - failover clustered SQL database backup is done by using the Virtual hostname for the clustered SQL instance. This will backup the database(s) on the active node.
Steps to backup clustered SQL database is covered in NBU for SQL Admin Guide.
Be sure to work with the SQL DBA to configure SQL database backups.
(Links to manuals can be found in Handy NBU links in my signature.)