2-1

1

If you meant something else by "2-1" (e.g., a textbook problem number, a base other than 10, or a larger expression), please share the full context and I’ll tailor the explanation. For quick practice, here are a few similar variants:

• 3 - 2 = 1
• 1 - 2 = -1
• 10 - 1 = 9
• In any base b > 2, 2_b - 1_b = 1_b