Answer:
Not all teachers will give extra credit assignments. You need to ask each teacher whether you can get credit for extra work, and if so, what should you do for extra credit.
If you just want extra practice work, there will be questions and work in each chapter of your textbook - you can also go online or to the library and look up other textbooks on the same subject and work those problems.
Many teachers will give extra credit for a good written report, especially if you include pictures or diagrams.