21. [10] Evaluate $\sum_{i=1}^{\infty} \frac{(i+1)(i+2)(i+3)}{(-2)^{i}}$.
Solution: This is the power series of $\frac{6}{(1+x)^{4}}$ expanded about $x=0$ and evaluated at $x=-\frac{1}{2}$, so the solution is 96 .