Write an expression to represent twice the difference between a number and 9

Based on what i know about mean, this doesn't add up.

F(n + 1) = f(n) – 8. if f(1) = 100, what is f(6)?

Asurvey of 45 teens found that they spent an average of 25.6 hours per week in front of a screen (television, computer, tablet, phone, based on the survey’s sample mean, which value could be the population mean? 2.3 hours 27.4 hours 75.3 hours 41.5 hours