The population of Whoville has been decreasing at a rate of 0.8% per year since Dr. Seuss passed away in 1991. If the population was 13,500 at the beginning of 2005, which expression gives its population at the end of 1998?

Well, if your asking from 2005 back to 1998... Starting from 13,500 take back 0.8% each year, in this case 7 years, it would be about 2831.