<span>You almost got it. Be careful with how the question is worded. A "common difference" between these sequential numbers already indicates subtraction. So you have to answer with what number is being subtracted from each number in order to equal the next. 9 - 10 is -1. Next, -1 - 10 is -11. Thus, the common difference between successive terms here is 10. Next, the difference between -11 and -21 is 10. You could continue the sequence: -21 - 10 = -31 and so on.</span>
