If you find that you cannot remember the answer to a question that you know you studied, try to imagine the place where you learned it. If you studied the material in the library, imagine the library. Try to recreate the entire context of where and how you learned the material: visualize the book, even the page where you found the material. Were there any diagrams on that page? What time of day were you studying the material?
Since the mind connects information in all sorts of ways, the contextual cues will give you a better chance at recalling what you learned.