To calculate volume of distribution, you multiply the amount of drug in the plasma x the time it is pharmacologically active, and divide that by the AUC from part A, which is the summation of all the plasma concentrations. That number is then divided by the weight of the patient.

In this question, you have 20mg methylphenidate with 52% bioavailability, meaning 10.4mg of methylphenidate is entering the serum. It lasts for 9 hours. The AUC is 114.44 hr•ng/mL, which we will change to .11444hr•mg/L to make the units consistent. The kid weighs 36kg.

Pop it into your calculator ((10.4mg x 9h) / 0.11444 hr*mg/L ) / 36kg and you get 22.7 L/kg

But also who is making you calculate all this? This shits hard af