The first thing you need to do is to make the expanded div initially hidden. Then to achieve what you want you can simply toggle both divs when the user presses the button. Like this:
<div id="expandedTerms" style="display:none">
Though it's preferrable to do the hiding in your CSS to avoid inline styling.
The script will look like this:
$( "#editButton" ).click(function() {
$( "#basicTermsRow" ).slideToggle( "fast" );
$( "#expandedTerms" ).slideToggle( "fast" );
});