What is the common difference in the sequence 9, 4, - 1, - 6?

A. 5

B. - 5

C. 9

D. - 11

(Am I dividing or subtracting?)

Use first differences. You are subtracting. Difference = Subtracting.

9 - x = 4

4 - x = - 1

-1 - x = - 6

Find for x.

9 - x = 4

- x = 5

x = - 5

Therefore the answer is B.

Proof:

9 - 5 = 4

4 - 5 = - 1

-1 - 5 = - 6