There are infinitely many rules that can generate this sequence. As imple one is
Un = 5n2 - 21n +72 for n = 1, 2, 3, ...
And then n = 4 gives U4 = 68
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The next number is 54. The sequence is generated by the rule: Un = (n4 - 11n3 + 51n2 - 107n + 102)/4 for n = 1, 2, 3, ... and U6 = 54.
216, 432, 864.... Just keep multiplying the number by 2.
It is 1000.