Kiera wanted to find the approximate number of historical books at the library she works at. she used the computer to randomly take a sample of 80 books and found that 6 of them were considered historical books. there are 21,360 books at this library. what is the best estimate of the amount of historical books at this library

The above is an estimation question. Estimation is when you judge the results obtained from a sample and make a rough calculation. The answer obtained might not be exactly right, its based on a judgement.

In this a sample was taken and out of 80 books, 6 of them were history books.
The assumption made then is that for every 80 books taken randomly , 6 of them will be history. Using this assumption ;
21 360 books in the library,
for every 80 books - 6 history books
then for 21 360 books - 6 / 80 * 21360
                                   =  1602 history books