Answer: Y = 3500, X = 16
Explanation:
Your maximum Y-value is 3000, or, the price of your car, so that could be the answer that's being looked for. However, it's always a good idea to leave a little extra space on the end of the graph so that you can show that your line ends instead of continuing onwards.
Your maximum X-value is 15, or, the number of months it would take to pay off the car (200 dollars per month, until you reach 3000 dollars), but again, it's always smart to leave a little buffer space so that you can see that the graph actually ends.
The answer your teacher wants is probably that the maximum Y-value should be 3000 and the maximum X-value should be 15, but if you have to actually graph this equation yourself and want it to be understood that your graph is a line segment and not a line (the difference being that a line is infinite and a line segment has two ends), you will want to show that by not ending the graph at the same spot as the line segment and instead extending it a little further.