In most courses in math and the sciences, you will find both subject material and example problems scattered throughout the chapters. Today, students are quite busy and rarely read the texts that their assignments come from. The unfortunate side effect: most times students lose context for each of the concepts being discussed, which can cause extreme confusion when trying to combine multiple concepts from a chapter into a single problem.
Not all the ways of old in school/education are outdated. This lost art of reading through the chapter’s example problems can help give clarity on when and how certain formulas should be used. Often times, 75% of the homework assigned will be related to these example problems in some way. It is only the challenge questions at the end of the section that require more than what can be gleaned from reading the example problems. Make sure you read these example problems to gain more confidence in, as well as comprehension of, the concepts that need to be used together in order to effectively solve problems.
If you cannot seem to grasp how to attempt a problem by reading these example problems scattered throughout the text, then try consulting your notes for any little extras the teacher may have added outside of the text. Though this is rare, it is becoming more common as teachers are customizing their courses to include homework outside texts.