Hello! 9 - 2.5 is 6.5. 2.5 - (-4) is 6.5. And so on. If you subtract two terms, the common difference between the successive terms is 6.5, because you subtract 6.5 from a term each time. When it comes to finding the difference between two terms, all you have to do is subtract two numbers and do this a couple of times to find the common difference.