HOW DO I GET MY PREVIOUS MATRIC EXAM NO
How Do I Get My Matric Examination Number?
It should be on your certificate if you have already taken the exam, or on the other literature you were given when you registered. If you can't find it there, you should go to the website of the body that is awarding your matric (I don;t know which country you are in, so can't link you to the website) and use their Contact Us section to find help.
You can find your matric exam no either from your certificate or visit relevant site of board where you appeared for exam