* Identify the first three significant digits: They are 2, 1, and 9.

* Look at the fourth digit: It's 7, which is greater than or equal to 5.

* Round up the third digit: Since the fourth digit is 7, we round the 9 up to 10. This carries over to the second digit, making it 2.

Therefore, 21970 rounded to 3 significant digits is 22000.