I have two tables both on the same sheet, firstly I want to compare two
columns (A and J) and get any missing values (show the answers in Column M),
then I want to compare the "sum" of two columns (A&B) from one table with two
columns (J&K) in the second table to show the differences (show the answers
in Column N).

Column A Column B ------ Column J Column K ------ Column M Column N
712 AB 712 AB
714 713 NA
713 AB 713 NA
719 717 SA
714 AB 715 AB
715 AB 716 AB
716 AB 717 SA
717 AB 718 AB
718 AB 720 AB
719 SA 721 AB
720 AB 722 AB