And what is their address?
SELECT address FROM patient WHERE SSN NOT IN  (  SELECT T1.patient FROM Prescribes AS T1 JOIN Medication AS T2 ON T1.Medication   =   T2.Code WHERE T2.name   =   'Procrastin-X'  ) 
their address refers to selecting the 'address' column; patients not prescribed 'Procrastin-X' medication involves a subquery excluding patients based on a join between 'Prescribes' and 'Medication' tables where the medication name is 'Procrastin-X';