If mr. jernigan refuses to rent the land to another farmer, then what will be his accounting costs from farming corn himself on his land? what will be his economic costs?

His accounting costs will be explicit costs - the cost to buy the corn, the cost to maintain the land, to maintain the crops, harvest the crops, to transport them to the market. Economic costs include the implicit costs - the rent that Mr. Jernigan could be earning if he rented the land to another farmer, and is not earning now.