I was so glad to get your letter. Thanks a lot for writing me often.
You asked me what should you do if you failed a maths exam at school. To my mind, you shouldn't hide it from your parents, because they are the one who can understand and empathize you. Trust them: just tell the truth and promise that it won't ever happen again. Anyway, nothing can't be changed. Just study a lot, and retake exam well.