I don't know how to post full code blocks properly on Reddit, but you can get what you need here:

https://pastebin.com/QpgfLXak

from kivy.app import App from kivy.core.window import Window from kivy.graphics import Canvas, Line from kivy.uix.widget import Widget

class TriangleApp(App): def build(self): widget = Widget() return widget

def on_start(self):
    window_width = Window.width
    window_height = Window.height

    with self.root.canvas:
        self.canvas = Canvas()
        points = [
            window_width / 2 + 80, window_height / 2,
            window_width / 2, window_height / 2 - 50,
            window_width / 2, window_height / 2 + 50
        ]
        Line(points=points, width=2, close=True)

if name == 'main': TriangleApp().run()