Here is some code.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=7, IE=9, IE=10">
<meta name="viewport" content="initial-scale=1, maximum-scale=1,user-scalable=no"/>
<title>Testing</title>
<link rel="stylesheet" type="text/css" href="http://serverapi.arcgisonline.com/jsapi/arcgis/3.5/js/dojo/dijit/themes/claro/claro.css">
<link rel="stylesheet" type="text/css" href="http://serverapi.arcgisonline.com/jsapi/arcgis/3.5/js/esri/css/esri.css" />
<style>
html,body {
height: 100%;
width: 100%;
margin: 0;
padding: 0;
}
#map {
overflow: hidden;
height: 100%;
width: 100%;
}
#toolsDiv {
position: absolute;
left: 60px;
top: 20px;
z-index: 2;
}
</style>
<script type="text/javascript">var djConfig = {parseOnLoad: true};</script>
<script type="text/javascript" src="http://serverapi.arcgisonline.com/jsapi/arcgis/?v=3.5"></script>
<script type="text/javascript">
var map;
require(["dojo/ready"], function(ready) {
ready(function() {
require(["esri/map"], function(Map){
map = new Map("map", {
basemap: "topo",
center: [-98.57, 39.82],
zoom: 4
});
})
});
});
function exportMap() {
require(["esri/geometry/scaleUtils", "dojo/dom"], function(scaleUtils, dom) {
var extent = scaleUtils.getExtentForScale(map, dom.byId("scaleBox").value);
var url = "http://services.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer/export?bboxSR=&layers=&layerdefs=&size=&imageSR=&format=png&transparent=false&dpi=&time=&layerTimeOptions=&f=image"
var bbox = "&bbox=" + extent.xmin + "," + extent.ymin + "," + extent.xmax + "," + extent.ymax
window.open(url + bbox);
});
}
</script>
</head>
<body class="claro" style="width:100%; height:100%">
<div id="map" style="width:100%; height:100%">
<div id="toolsDiv">
<input id="scaleBox" type="text" name="scaleBox" data-dojo-type="dijit/form/TextBox" value="Enter scale. eg. 10000" />
<button id ="goButton" type="button" name="goButton" data-dojo-type="dijit/form/Button" onclick="exportMap()">Export</button>
</div>
</div>
</body>
</html>