I'm sorry you haven't passed your exit exam. It's great to know that you are concerned and attempting to find resources to aide in this process. The best book, used for most exams is called Math Review for Standardized Tests. This book helps you learn the concepts needed for these kind of exams.
I'm sending you a link where you can purchase the book. Or, you can see if the library has it in their test prep sections.
I was in the same boat back during my senior year too. I had to actually go to summer school for math and then I finally passed it. I wish you luck. If its not too late try to get into a summer class or try a tutor(a fellow student who is good at math or a trusting teacher.)
Nowadays, there are a lot of schools that offers free education online. But it is best if you could ask for a friends suggestion to make sure that the school offers quality education and is accredited.