what is the total dosage of bag that was prescribed to patient 14397 since 34 months ago?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 14397 ) and prescriptions.drug = 'bag' and datetime(prescriptions.startdate) >= datetime(current_time,'-34 month')