The sequence \((a_n)_{n \ge 1}\) starts \(-1, 0, 2, 5, 9, 14\ldots\) and has closed formula \(a_n = \dfrac{(n+1)(n-2)}{2}\text{.}\) Use this fact to find a closed formula for the sequence \((b_n)_{n \ge 1}\) which starts \(4, 10, 18, 28, 40, \ldots\text{.}\)