To find the average rate of change (average) between two years in the given situation you use the next formula:
[tex]\begin{gathered} \text{Average}=\frac{f(b)-f(a)}{b-a} \\ \\ Being\text{ a and b the years and f(a) and f(b) the sales in the corresponding year:} \\ \\ \\ \end{gathered}[/tex][tex]\begin{gathered} \text{Betw}een\text{ }2014\text{ and 2}015\colon \\ a=2014 \\ b=2015 \\ \\ f(a)=364 \\ f(b)=394 \\ \\ \text{Average}=\frac{394-364}{2015-2014}=\frac{30}{1}=30\text{millions of dolllars/year} \end{gathered}[/tex][tex]\begin{gathered} \text{Between }2014\text{ and 2017:} \\ a=2014 \\ b=2017 \\ \\ f(a)=364 \\ f(b)=418 \\ \\ \text{Average}=\frac{418-364}{2017-2014}=\frac{54}{3}=18\text{millions of dolllars/year} \end{gathered}[/tex]