namespace FirstApp
{
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
        }
        private void Calculate
        {
            Double Num1;
            Double Num2;
            Answer = (Num1 + Num2).ToString();
        }
    }
}

<FlexLayout>
    <Entry Text="{Binding Num1}"></Entry>
    <Entry Text="{Binding Num2}"></Entry>
    <Button Text="Go" Clicked="Calculate"></Button>
    <Label Text="{Binding Answer}"></Label>
</FlexLayout>