<div id="d8" class="menutitles ng-scope" ng-repeat="title in list">
     <div class="right" ng-click="showsubmenu($event)">+</div>
     <div class="left" ng-click="showsubmenu($event)">Unit 9</div>
</div>

$scope.showsubmenu=function(obj)
{
    alert(obj.target.parent.attributes.id)
}

alert(obj.target.parentNode.id);

<div id="d8{{$index}}" class="menutitles ng-scope" ng-repeat="title in list">
    <div class="right" ng-click="showsubmenu()">+</div>
   <div class="left" ng-click="showsubmenu()">Unit 9</div>
</div>

<div id="d8" class="menutitles ng-scope" ng-repeat="title in list">
    <div class="right" ng-click="showsubmenu()">+</div>
   <div class="left" ng-click="showsubmenu()">Unit 9</div>
</div>

function showsubmenu($event){
    $($event.target).parent();

}