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

The terms of this arithmetic sequence are: a n = 9, a n+1 = 2.5, a n+2 = -4, a n+3 = -10.5, a n+4 = -17. Also we know that: a n+1 = a n + d, where d is the common difference. 2.5 = 9 + d, d = 2 - 9.5, d = - 6.5. We can check this 2.5 + ( - 6.5 ) = - 4, - 4 +( - 6.5 ) = -10.5, - 10.5 + ( - 6.5 ) = - 17. The common difference is - 6.5.