when they visited the hospital last time what was the first dose of requip xl prescribed to patient 1912?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 1912 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and prescriptions.drug = 'requip xl' order by prescriptions.startdate asc limit 1