Which students that are in the marines have been absent for 6 months?
in the marines refers to organ = 'marines'; absent for 6 months refers to month = 6
SELECT T1.name FROM longest_absense_from_school AS T1 INNER JOIN enlist AS T2 ON T1.`name` = T2.`name` WHERE T2.organ = 'marines' AND T1.`month` = 6