1. The question "What topic is this under?" is asking to identify the subject area or category of a problem or concept. 2. In mathematics and related fields, topics are usually categorized by the type of problem or the methods used, such as algebra, geometry, calculus, physics, etc. 3. Since no specific problem or concept was provided, it is not possible to determine the exact topic. 4. If you provide a specific math problem or concept, I can help identify the topic it belongs to.