Solve the Problem $30 \mathrm { mph }$ And Above She Notices That Traveling at an Average Speed Of

Multiple Choice

Solve the problem. Write all linear equations in slope-intercept form.
-A driver wants to gauge the fuel efficiency of her vehicle at speeds of $30 \mathrm { mph }$ and above. She notices that traveling at an average speed of $40 \mathrm { mph }$ results in a rating of $36 \mathrm { mpg }$ , whereas at an average speed of $45 \mathrm { mph }$ , her car rates $21 \mathrm { mpg }$ . Find a linear equation to model the gas mileage y for an average speed of $x \mathrm { mph }$ . Find the slope-intercept form of the equation of the line.

A) $y = - 3 x + 156$
B) $y = \frac { 1 } { 3 } x + 156$
C) $y = \frac { 1 } { 3 } x + 84$
D) $y = - 3 x + 84$