Answer:
what she sells each t-shirt for.
Explanation:
if f(x) is the profit and x=# of t-shirts,
the rate of change is 8, which is what she sells each t-shirt for.
The 100 that is subtracted, is the money that she spent on materials.
For eg, if she sells 8 t-shirts she makes $64, but she spent $100 before and didn't gain any profit, having not made up $36 from what her original spending was.