Depreciation assets capital gain

a firm purchase land in 1989 and construction part godown in 1990 and construction building in 2002. claiming depreciation in year by year book's of account and also income tax. in 2012 firm stopped business and also not use in entire land and building and also not filed income tax return after that now the firm sales the land and building how to calculate the capital gain it's short term or long term or index method