when was patient 7698 last prescribed tigecycline, on the first hospital encounter?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 7698 and admissions.dischtime is not null order by admissions.admittime asc limit 1 ) and prescriptions.drug = 'tigecycline' order by prescriptions.startdate desc limit 1