when was the first time that patient 2700 was prescribed with dexamethasone in 12/2100?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 2700 ) and prescriptions.drug = 'dexamethasone' and strftime('%y-%m',prescriptions.startdate) = '2100-12' order by prescriptions.startdate asc limit 1