CSS Problem: How to control the z-index?

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CSS

Hi guys,

I have in my website a menu and down to it there is an auto-playing slider, the menu has long sub-menus, and the problem when I open the menu I cannot see the whole list of sub-menus; the moving slider hides it. It just come over the menu.
I tried to edit the CSS, the z-index attribute, but still the same problem.
I want the sub-menu over the slider.

Here is the picture:

<img>http://im24.gulfup.com/2012-07-31/1343690359371.png</img>


Here are the design code of menu

<div id="menus" style="z-index: 10px;">
        <ul class="menus">
            <li><a href="index.aspx"><span>Home</span></a></li>
            <li><a class="parent"><span>Educational Plan</span></a>
                <ul>
                    <li><a href="#" class="parent"><span>Education Faculty</span></a>
                        <ul>
                            <li><a href="#"><span>Curricula & Teaching Methods Dep.</span></a></li>
                            <li><a href="#"><span>Educational Mgmt. Supervision Dep.</span></a></li>
                            <li><a href="#"><span>Islamic Studies Dep.</span></a></li>
                            <li><a href="#"><span>Educational Mgmt. Supervision Dep.</span></a></li>
                            <li><a href="#"><span>Islamic Studies Dep.</span></a></li>
                            <li><a href="#"><span>Lingustics Dep.</span></a></li>
                            <li><a href="#"><span>Literatures Dep.</span></a></li>
                        </ul>
                    </li>
                </ul>
            </li>
        </ul>
    </div>

and this is the design code of slider

<div id="prew_img" style="z-index: 3px;">
            <ul class="round">
                <li>
                    <img src="images/header1.jpg" alt="" /></li>
                <li>
                    <img src="images/header2.jpg" alt="" /></li>
                <li>
                    <img src="images/header3.jpg" alt="" /></li>
                <li>
                    <img src="images/header4.jpg" alt="" /></li>
                <li>
                    <img src="images/header5.jpg" alt="" /></li>
                <li>
                    <img src="images/header6.jpg" alt="" /></li>
            </ul>
        </div>