Your answer is not correct. You can see that is wrong by trying to use it to answer the question "what is the probability that 3x-2y = 5". There is no way to input this information into your function to get an answer.
Starting over, define a single variable Z = 3X - 2Y
You want the PMF of Z
ie, to get a particular value of Z we cna have any value of X, provided that Y = (z+3x}/2
The last step is called the method of convolutions, which hopefully you have seen before if you were set this problem.