What is the next number in this sequence 7 21 8 72 9.....I need to answer this question for a test and math has been a long time ago. What is the best way to relearn math sequences and probability and statistic.

7, 7×3, 8, 8×3

^{2}, 9, 9×3^{3},....n, followed by n×3^{n-6}for example
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