Yes. 3n2/12n3 = 1/4n
There are infinitely many possible answers. the simplest polynomial is Un = -n4 + 12n3 - 47n2 + 78n - 41
The rule, using a polynomial function of the lowest order is Un = (12n3 - 36n2 - 145n + 413)/25
The next number is 18. This is obtained by fitting the quartic polynomial t(n) = (-n4 + 12n3 - 47n2 + 84n - 36)/4 for n = 1, 2, 3, etc.
The quartic: t(n) = -n4 + 12n3 - 44.5n2 - 24.5n + 356 fits the data for n = 1, 2, 3, ... Putting n = 6 gives t(6) = -97 NB: A more popular version of this question has 129 as the third number.
Given any number it is easy to find a rule based on a polynomial of order 5 such that the first five numbers are as listed in the question and the next is the given number. There are also non-polynomial solutions.The simplest polynomial rule isUn = (n4 - 12n3 + 50n2 - 69n + 36)/6 which gives the next number as 21