Capital gains

As per new  Indexation  Chart  from 2001-2002  =  100

For  year  of   property sold   2020-21                  =   301

You have purchase in 1984 , so as new Index chart  you have take base year as 2001 

Just  confirm  form  Income Tax web site - www Incometax .gov.in 

No , you have to find out  the value of  property  from  registered  valuer in 2001  and then consider Index value  100  for year 2001-2002 as a base .