Implementing INotifyPropertyChanged in xamarin.forms

I am doing a cart in xamarin.forms, in my cart page there is a list view with data. Each of the cell containing a button to select the count of item and amount. In the cart view there is a label for grant total, my problem is the grant total is not updating while deleting a item from the cart . The calculation method is on item adding view cell . i know that i need to implement inotifyproperty for this, but i don't know how to do it

I have a base view model which inherit inotifyproperty which contain the event also

public class BaseViewModel : INotifyPropertyChanged{
     private double  _price;
    public double Price{
        get { return _price ; }
        set{_price =value;OnPropertyChanged("Price");} } 
 protected virtual void OnPropertyChanged(string propertyName){
        if (PropertyChanged != null {
            PropertyChanged(this,
                new PropertyChangedEventArgs(propertyName));}}

public BaseViewModel(){
        App.Instance.ViewModel = this;
        TempList = TempList ?? new ObservableCollection<cm_items>();
        this.Title = AppResources.AppResource.Cart_menu_title;
       this. Price = CartCell.price;}

public static double Calculate_price(){
            App.Instance.ViewModel.Price = price;
            price = 0;
            var s = App.Instance.ViewModel.TempList;
            var itm_cnt = App.Instance.ViewModel.TempList.Count;
            for (int i = 0; i < itm_cnt; i++){
                var val1 = s[i].cnt;
                var val2 = s[i].item_price;
                price += Convert.ToDouble(val1 * val2); }
            return pri