Series 13 40 85 148 229
1st diff 27 45 63 81
2nd diff 18 18 18
There's a pattern because the 2nd differences appear to be the same.
Let's take 4 off each of the terms in the series:
9, 36, 81, 144, which are 1×9, 4×9, 9×9, 16×9.
1, 4, 9, 16 are the squares of the natural numbers. Also 229-4=225=25×9.
The formula for the series is 9n2+4 where n=1, 2, 3, 4, ...
When n=10, the term is 904, because 102=100. So x=904.
For N, x=9N2+4.