One possible answer is 494.
This uses the cubic function (-29x3 + 256x2 - 401x + 218)/2
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A single number, such as 21357, does not define a sequence.
It is not possible to identify a sequence when given only one number, as in this questions.
If you mean: 9, 19, 39, 69 then the next number in the sequence is 109
5
121498