Is it enough to have only full backup and transaction log backup only for a good backup plan?
My problem is what is the use of differential backup every 6 hours if we are taking a transaction log backup every 15 minutes?


Differential backup is used to decrease the Recovery time object(RTO), it gives you facility of restoring backups quickly. Differential backup includes all the changes made since last full backup, or full backup which started the differential chain. Differential backup is not affected by log backup so in event of disaster you would have to restore(in your case)


12 PM Sunday full backup
Differential backup taken before the disaster strike. Note this is just one diff backup you have to restore no matter how many you have taken because its cumulative
The log backups after diff backup (in point 2) stopping just before the disaster struck.