A faulty volume down key is the issue try turning your volume down when it's started up if it isn't working then you have a faulty component simply ope the back and carefully remove the small ribbon connector just behind the volume key the swipe down and select switch off safe mode then jobs done all it means is you have to use on screen volume controls

last app you installed its i m don't undastend.which last app installed tall me?

i m hard reset samsung tab 3 but still safe mode .so how to out safe mode please give solution.

Usually, restarting the device gets you out of safe mode. But if not, there may be app/s that's causing problem. Usually it's the last app you installed, i suggest doing it.

Let me know if the problem is fixed after.