I have 7 mo old baby. She has been constipated. need dosage of miralax asap.dr forgot to give and they closed?

We went to after hours pediatrics. Dr gave prescription for miralax. I asked her how much to give. She said directions will be given to me at the pharmacy. At the pharmacy, after i've waited 30 min,around 10 pm, pharmacist told me that Dr. didn't give directions on dosage and he has no idea what it is for 7 month old baby. Also he thinks this medicine is not for 7 month old. By that time Pediatric after hours closed till Wednsday. I came home, called my pediatrician. Left several messages and nobody would call back. Help. Baby is constipated. need dosage and water amount to mix Miralax with for 7 month old, 17 lb. Thank you so much