The table below gives the annual sales $($in millions$)$ of a product.

$\backslash$table$[[$year$,2015,2016,2017,2018,2019,2020,2021,2022,2023],[$sales$,180,225,260,285,300,305,*300,285,260]]$

What was the average rate of change of annual sales

a$)$ between $2016$ and $2017?$

millions of dollars$/$year

b$)$ between $2016$ and $2023?$

millions of dollars$/$year