Use induction to prove that for every integer $\displaystyle n \geq 4$, $\displaystyle 3^n > n^3 $.

Here's the solution:

http://img32.imageshack.us/img32/5042/14616641.gif

I don't understand where the term $\displaystyle k^3+3k^2+4k$ came from in the middle line. Could anyone explain?