A formula that approximates the dosage of a drug prescribed for children is shown below. In the​ formula, Aequalsthe ​child's age, in​ years, Dequalsan adult​ dosage, and Cequalsthe proper​ child's dosage. The formula applies for ages 2 through​ 13, inclusive. Use​ Young's rule to find the difference in a​ child's dosage for a 9​-year-old child and a 7​-year-old child. Express the answer as a single rational expression in terms of D. Then describe what your answer means in terms of the variables in the model. ​Young's Rule: Upper C equals StartFraction DA Over Upper A plus 12 EndFraction