Which language are you developing in?
Javascript:
http://help.arcgis.com/en/webapi/javascript/arcgis/help/jsapi_start.htm#jsapi/map.htm
Under Map Navigation look at center and zoom
Flex:
Possibly http://resources.arcgis.com/en/help/flex-api/apiref/com/esri/ags/utils/WebMercatorUtil.html
Check out example at bottom
Silverlight:
http://help.arcgis.com/en/webapi/silverlight/apiref/ESRI.ArcGIS.Client~ESRI.ArcGIS.Client.Map~ZoomTo...
Regards,
Tom