Finding an index in an ObservableCollection

This may be very simple but I have not been able to come up with a solution. I am have a: ``` ObservableCollection<ProcessModel> _collection = new ObservableCollection<ProcessModel>(); ``` This collection is populated, with many ProcessModel's. My question is that I have an ProcessModel, which I want to find in my _collection. I want to do this so I am able to find the index of where the ProcessModel was in the _collection, I am really unsure how to do this. I want to do this because I want to get at the ProcessModel N+1 ahead of it in the ObservableCollection (_collection).