I realise you've probably gotten past this question, but I'm throwing it out for all future askers of this question ¯\_(ツ)_/¯
C) ƒ(x) increases by a factor of 3
ƒ(x) increases by a factor of 3
f(4) = 3^4
f(5) = 3^5
f(5) /f(4) = 3^5 /3^4 = 3^5−4 = 3^1 = 3
The value of ƒ(x) changes from 81 to 243 over the interval from x = 4 to x = 5.
This is an increase by a factor of 3.
