Three more tips:
1. With long LaTeX expressions like that, you need to be careful to not type 50 characters without a space, because vBulletin will insert a space every 50 characters and break the LaTeX code.
2. When you want the math expression to appear by itself on a new line, use tex tags instead of itex, and don't type newlines before and after the tags. You will get [tex]t = \frac{2*A_{t}\left(\sqrt{z_{1}}-\sqrt{z_{2}}\right)}{C_{d}*A_{o}*\sqrt{2g}},[/tex] instead of
[itex]t = \frac{2*A_{t}\left(\sqrt{z_{1}}-\sqrt{z_{2}}\right)}{C_{d}*A_{o}*\sqrt{2g}}[/itex].
Hit the quote button to see exactly what I did.
3. If you want a multiplication symbol, use \times or \cdot instead of *.