(related to Problem: Sums of Falling Factorial Powers)
Using the formula for the falling factorial powers from antidifferences of some functions, we get
$$\sum_{x=3}^{16}x^{\underline{3}}=\frac{x^{\underline 4}}{4}\;\Rule{1px}{4ex}{2ex}^{17}_{3}=\frac{17\cdot 16\cdot 15\cdot 14-3\cdot 2\cdot 1\cdot 0}{4}=14280.$$