There are many possible answers but a relatively straightforward one is
Un = 2n2 - n + 3 for n = 1, 2, 3, ...
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The rule for the sequence 4, 9, 18, 31, 48, 69 is that each number is obtained by adding a consecutive odd number starting from 3.
Starting with 4, we add 3 to get 7, then add 5 to get 12, then add 7 to get 19, then add 9 to get 28, and finally add 11 to get 39.
Therefore, the next number in the sequence should be 39.