How do I graph a cost function like #C(x) = 3x + 20,000#?
I think that your function can be used to model a service (say a health insurance) where you pay a fixed value (20,000) to join the program and 3 for each medical visit.
Your function is a linear function and is represented, graphically, by a straight line.
Considering an example you can immagine that it represents a medical insurance.
You can now plot these points on your graph.
So, graphically you have: