Answer: C. f(x) = |x|
Step-by-step explanation:
Answer is in the photo. I can't attach it here, but I uploaded it to a file hosting. link below! Good Luck!
tinyurl.com/wtjfavyw
I positive 15 and negative 15, in effect the absolute value function just removed negative signs
Find the unit rate, which is also known as slope, constant rate of change!

First, add all the numbers together. 
Divide by the amount of numbers. 
is the closest, so that is the answer.