What is the nth term for 2 11 26 47 74 107?

There are infinitely many polynomials of order 6 (or higher) that will give these as the first six numbers and any one of these could be "the" rule. Short of reading the mind of the person who posed the question, there is no way of determining which of the infinitely many solutions is the "correct" one.
In this particular case, the simplest solution isU(n) = 3*n^2 - 1 for n = 1, 2, 3, ...