Answer
The slope is $44$ (million dollars of income) per (billion dollars in sales).
For every increase of a billion dollars of net sales, Amazon earns another ${{\$}} 44$ million in income.
Work Step by Step
The unit of slope in
$y=mx+b$
is (unit of y) per (unit of x)
Slope is calculated as (change in y)/(change in x).
Here, $I=44S-220$
the slope is $44$ (million dollars of income) per (billion dollars in sales).
So, for every increase of a billion dollars of net sales, Amazon earns another ${{\$}} 44$ million in income.
