Capital gain on sale of agriculture land

I had an agriculture land received as ancestral property from my father. The land belonged to my Father since 1940. The Land has been allocated to me as partition of the property in the family in 1969.

I recently sold this land . Let me know followings.

1) How the capital gain tax is calculated. Indexation available from 1981

2) To avoid paying CG tax, can I purchase a residential / commercial property ?