Answer:
When x <= 8000
The cost remains constant at 0.35 when x increases from 0 to 8000
The slope of cost function over this part is 0
When 8000 < x <= 20000
The cost remains constant at 0.75 when x increases from 8000 to 20000
The slope of cost function over this part is 0
When 20000 < x <= 42000
The cost decreases when x increases from 20000 to 42000
The slope of cost function
m= -5 × 10^-6