2-1

Could you clarify what you mean by "2-1"? It could refer to things like:

• Section 2.1 of your textbook/course
• Problem 2.1 from an assignment
• A quick arithmetic 2 − 1

Please share:

• The subject/course (e.g., Calculus, Linear Algebra, Statistics)
• The textbook or notes (if any)
• The full problem statement or a screenshot

I can then give you a step-by-step solution, a summary of the relevant concepts, or a set of practice problems for that topic.