The sequence follows the pattern of $2n^2 + 3$ for $n = 1, 3, 5, 7, 9, 11$. Specifically: $2(1)^2+3=5$, $2(3)^2+3=21$ (mismatch), or more simply, the differences are 20, 36, 52, 68. The difference between differences is a constant 16. Adding $68+16=84$ to 181 yields 265.