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It appears that you're using "Building Java Programs: A Back to Basics Approach, by Stuart Reges and Marty Stepp." Their use of custom graphics "helper" classes that do most of the work required to draw graphics. The DrawingPanel class that you're using is not core Java, it's a class the authors of the book wrote to "help" students using the book.
Most here know standard or core Java and the classes distributed by Oracle with the Java SDK. To help you, we'd have to learn the Reges and Stepp approach or already know it. Be patient, and someone may come along who can help, but I haven't seen this approach before on this forum, so it may be a long wait.