Does the problem tell you how much the monthly charge was? Look for that. It may be in the instructions or in a previous question. With the information you gave us, it is only possible to determine how much the cell phone service cost over the course of 9 months, just as in the equation that Jason did, but you need to have a second variable in order to determine how much Mel started with. Otherwise, you would just say that Mel started with $0, but that is probably not what the question is looking for.
If that is the missing information, here is how you work out that problem:
Let's say that the monthly charge is $30 (I am making up this number. You can plug in the right number when you get it.)
Then you take 30 and multiply it by 9 months (30 x 9 = 270)
Then take the total amount that Mel paid (here you told us that it is $390) and subtract from that amount the total amount for 9 months worth of service, which we just calculated to be $270. (So, 390-270 = 120). If we use the 30 for our "x" variable, then Mel started off with $120 in costs when his service just began.