Snape sucked at working with others. He didn't like working with others and others didn't like working with him. But of course Professor Slughorn had to make the next assignment a 2-3 week thing done in groups of two. The assignment is gather ingredients to make a potion of your group's choice as long as it's a potion that has over 4 ingredients and it's dangerous to obtain. You were paired with Snape and so here you two are in the library