In order to continue hunting Ranrok, you’ll have to continue building your spell vocabulary. Professor Sharp wants to help you learn Depulso, the Banishing Charm, but he needs you to do some tasks first.
Professor Sharp has assigned you to drink three potions. Specifically, you will have to drink a Maxima and Edurus Potion at the same time, as well as a Focus Potion. You can choose to brew these yourself or simply buy them from J. Pippins Potions in Hogsmeade.
If you opt to buy the potions, plan to budget around 1100 galleons to afford all three. The Maxima and Edurus Potion are 300 each, while the Focus Potion sets you back 500.
On the other hand, for those who spent too much money on clothes or hate supporting local businesses, here are the recipes for each potion: