Discount on Bonds Payable

Question: discount on bonds payable has not been amortized for any part of 2014; the bonds are dated March 1, 2008 and mature on March 1, 2018 
Bands payable
275000cr.
Discount on bonds payable
6400dr.
I know the journal entries would be
interest expense dr.
Discount on bonds payable cr. 
can you please help me understand how to calculate this?