I can infer from context that you've not shown any work because you're stuck at the very beginning and have none to show. That's perfectly alright. The key words to pick out of the problem text are
arithmetic sequence (you might have seen it called an arithmetic progression). This gives you a good topic to search your class notes, textbook, and any handouts you may have been given for a refresher on this topic. If you're finding all of those insufficient, you might also try
this page from Math is Fun.
With that in mind, in order for the three given numbers to form an arithmetic sequence, there needs be some common difference. What expression, in terms of x, do you need to add to 3 - x to get x? What happens if you then add this same difference again to x? Using what you know about arithmetic sequences, what expression must this be equal to? Where does this all lead you? Go ahead and give the problem your best effort. If you get stuck again, that's fine, but when you reply back please include any and all work you've done on the problem, even including the parts you know for sure are wrong. Thank you.