You have three years from the due date of the tax return to file for a refund. So you must file a 2015 return no later than April 15, 2019, to claim the refund. You can find prior year tax forms from the IRS at https://www.irs.gov/forms-pubs/prior-year.
A statutory method of depreciation allowing accelerated rates for most types of property used in business and income-producing activities during the years 1981 through 1986. It has been superseded by the modified accelerated cost recovery system (MACRS) for assets placed in service after 1986.