Khan Academy is an amazing free resource available for our students (and for us as well). While it can be overwhelming, a simple way to use it would be to send review questions to your students. What is great about it is that the students can take the assessment (usually between 4-7 questions), receive assistance if needed, and then see their score. What I especially like is the fact that they can click on the score and see their answers to the questions as well as retake the assessment and get a higher grade!
Embedded below is a 1 minute and 37 second YouTube tutorial showing you the necessary steps.