21,000 is already rounded. So your answer is 21,000.
if you had been given 21,324; if we round it to the nearest thousand, it will be 21,000 as the nearest thousand of 21325 is 21000.
How: if the last three digits form a number less than 500, then round it to the same thousand as the number.
if you had been given 21,684; if we round it to the nearest thousand, it will be 22,000
here, the last three digit, 684 is more than 500. So it will be rounded to next thousand of the number. So it becomes 22,000.