## Thinking Mathematically (6th Edition)

RECALL: In the linear function $f(x) = mx + b$, $m$ is the slope or (average rate of change in $f(x)$ or $y$ for every unit increase in $x$), and $b$ is the y-intercept (value of the function when $x=0$). The equation that models the annual cost has a slope of $0.61$ and a y-intercept of $3500$. A y-intercept of $3500$ means that when the car is not used, the annual cost is $\$3500$. A slope is an average rate of change, so a slope of$0.61$means that for every mile driven, the annual cost increases by$\$0.61$. Therefore, the given statement makes sense.