From what I understand with the problem, you are only asked to find the value of x which is the number of months. So, you don't really need a graph to answer this question. Just solve this like any other problem in algebra.
100 + 36x = C, where the maximum value of C is $352
100 + 36x = 352
36x = 352 - 100
36x = 252
x = 7 months
Therefore, x should be at most 7 months.