Retrieve Multiple records using XrmSvcTool in CRM 2013 javascript RetrieveMultiple function

function Calculations()// to get marks of all student courses marks related to student
{
var stdId = Xrm.Page.data.entity.getId();
if (stdId != null)
{
        var EntityNameP, ArrayNamesP, ArrayValuesP;
        ArrayValuesP = stdId;
           var _query = "?$filter=cms_Student/Id eq (Guid'"+stdId+"')";
        var obj = RecRetrieveMultiple(_query, "cms_studentcourse");
        var entityNodesP = obj;
               var TotalPoints = 0;
        for (var i = 0; i < entityNodesP.length; i++)
        {
            var entityNode = entityNodesP[i];
            var Node = entityNode.cms_ObtainedMarks;//get simple field
             var level = entityNode.cms_level.Value;
            var courselevel = entityNode.cms_CourseId.Name;//get lookup value from child entity
            var courselevel1 = entityNode.cms_CourseId.Id;//get lookup id from child entity
             var ResultNode = (Node == null) ? 0 :  parseFloat(Node);
            TotalPoints = parseFloat(TotalPoints) + parseFloat(ResultNode);//sum of all student courses marks
        }       
    }
}
function RecRetrieveMultiple(_query, _entityname){

var contacts;
XrmSvcToolkit.retrieveMultiple({
    entityName: _entityname,
    async: false,
    odataQuery: _query,
    successCallback: function (result) {
        contacts = result;
    },
    errorCallback: function (error) {
       alert("Error");
    }
});
var _result = contacts;
return _result;
}