Solve the Problem $f ( x ) = \frac { 1 } { 2 } x + 89$

Multiple Choice

Solve the problem.
-A driver wants to gauge the fuel efficiency of her vehicle at speeds of 30 mph and above. She notices that traveling at an average speed of 30 mph results in a rating of 29 mpg, where as at an Average speed of 35 mph, her car rates 19 mpg. Find a linear function f(x) = mx + b whose input is
The speed in miles per hour and whose output is the gas mileage in miles per gallon.

A) $f ( x ) = \frac { 1 } { 2 } x + 89$
B) $f ( x ) = - 2 x + 31$
C) $f ( x ) = - 2 x + 89$
D) $f ( x ) = \frac { 1 } { 2 } x + 31$