The answer you are looking for is 8.3%.
To find this, you must find out how many students did not choose to voice their opinions. Simply add 17 and 5 to get 22, and subtract 22 from 24. This will give you 2 students who didn't voice their opinions. Then, divide 2 by 24. In doing so, you'll get 0.083333... which you multiply by 100 to get a percent. You'll be left with 8.3333... which rounded to the nearest tenth, is 8.3%.
I hope this helps!