what was the levetiracetam dosage that patient 83709 was last prescribed since 66 months ago?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 83709 ) and prescriptions.drug = 'levetiracetam' and datetime(prescriptions.startdate) >= datetime(current_time,'-66 month') order by prescriptions.startdate desc limit 1