What's the rule of 3 6 9 14 21? What's the next number

What's the rule of 3 6 9 14 21? What's the next number

9=3+6-0
14=6+9-1
21=9+14-2
X=14+21-3=32

2. What are the rules of numbers 3, 4, 6, 6, 9, 12, 18 and 21?
RT
In fact, 18 and 21 were left blank, but according to the students, this is the answer
In fact, I suspect that the students read the wrong answer

2. 3, 4 and 6 are 1 * 2, 1 * 3, 1 * 4 and 1 * 6 respectively
6. 9,12,18,21 are 3 * 2,3 * 4,3 * 6,3 * 7

Find the law, but use the formula containing n to express
The upper and lower numbers are a group of numbers
1 2 3 4 5…… n
-1 -3 -5 -7 -9…… （ ）

1-2n