Again, everything looks good up until this point:

You've lost the cubic power! I also think you mean

, not

. Try working those corrections back through and see what you get.

Here's a little hint for doing math in LaTeX: copy and paste. Whenever you're simplifying things, just copy the equation from the previous line, and make whatever changes you want. You'll make fewer mistakes that way than if you re-type out the whole equation. I do this all the time, and it's not only a mistake-saver, it's a real time-saver.