Answer
The slope is $0.16$ (billion ${{\$}} $ of expenses) per (billion ${{\$}} $ in sales).
For every increase of a billion dollars of net sales,
Amazon has another ${{\$}} 0.16$ billion in expense.
Work Step by Step
The unit of slope in
$y=mx+b$
is (unit of y) per (unit of x).
The slope is calculated as (change in y)/(change in x).
Here, $E=0.16S+0.2$
and the slope is $0.16$ (billion ${{\$}} $ of expenses) per (billion ${{\$}} $ in sales).
So, for every increase of a billion dollars of net sales,
Amazon has another ${{\$}} 0.16$ billion in expense.
