The official manual (ISBN-13: 978-1305964222 for the 5th edition) costs roughly $50–80 new or $20–40 used. This is the only 100% legal, safe, and accurate source. It includes explanations written by the authors themselves.
If your answer differs from the manual, don't just erase yours. Work backward from the manual’s solution to see exactly where your logic failed.
Every problem from the textbook is addressed.