What is the common difference between successive terms in the sequence

9,2.5,-4,-10.5,-17

Let's find the difference between two consecutive terms

2.5 - 9 = -6.5

-4 - 2.5 = -6.5

-10.5 - (-4) = -6.5

-17 -(-10.5) = -6.5

We can see that the common difference is -6.5

-6.5 is the common difference between successive terms in the sequence 9,2.5,-4,-10.5,-17

the common difference is negative because we subtract 6.5 to get the next term.