21-2

I’m not sure what you need with just “21-2.” Do you mean:

• 21 minus 2? If so, 21 − 2 = 19.
• 21 choose 2 (binomial coefficient)? That would be C(21, 2) = 210.
• A section/problem label (like chapter 21, problem 2)? Please share the exact problem statement.
• Something else (e.g., a modular arithmetic expression, a base notation)?

If you can provide the context or the full problem, I’ll help you solve it step by step.