The best way to do this is to look at the question, and see no matter what, we have to pay 200 dollars to start. After which, they charge 50 bucks a month. Knowing this, we can make a function using f(x). Let C(t)= cost. Included is that graph. So for these questions, we need to see that there is an independent and a dependent variable, and we need to see that cost is affected by time. Hope this helps.