It is vital for you to write step-by-step answers next to homework problems, and generally, all of the work that you do in math class. It will prove to the teacher that you successfully completed your work (without guessing), and if you forget a topic or how to complete a certain type of problem, you can look back in your notes to see how you did it previously, since you will have a step-by-step guide. You should also keep a hard copy of all work submitted in class. This is a good idea because it will be easily accessible for review, and if there is ever any question whether or not you completed an assignment, you will have physical proof.
Work Step by Step
Intermediate Algebra (Sixth Edition) by Elayn Martin-Gay Exercise Set 1.1: Tips For Success in Mathematics, Question 6.