I was solving an example problem using a different method using a formula linked below. The answer I ended up has an "x" in the denominator which isn't there in solution the book presents. I've failed to figure out where I made a mistake. Could you please help me? Thanks a lot.

**Book's solution:**http://img810.imageshack.us/img810/8342/examplek.jpg

**My Solution:**http://img717.imageshack.us/img717/2970/img0003wu.jpg

**Formula used:**http://img411.imageshack.us/img411/8596/exactequation.jpg

With best regards

PG