<Window x:Class="LineWithPolygon.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Polygon Points="20,20 20,200 250,300" Stroke="Red" StrokeThickness="3"/> 
    </Grid>
</Window>

Output :


wpf25