From the question given
Past studies have shown that only about 84% of the booked passengers show up
This can be taken as the probability of success (P)
Since we are told to find the probability that if Air-USA books 21 people, not enough people will show up
Then
In this case

We will use the formula below to find the probability that if Air-USA books 21 people, not enough people will show up

since we have the values of P and n


For the B part of the question
let us convert the answer obtained to percentage so we can compare it with 5% probability

Thus,
yes, the probability is low enough so that overbooking is low enough to be a concern.
For the part C of the question
since we have already converted to a percentage, then we will compare to 10%
Since

yes, the probability is low enough so that overbooking is low enough to be a concern.