Interesante ejercicio
<canvas width="600" height="400"></canvas>
<script>
var pantalla = document.querySelector("canvas");
var pincel = pantalla.getContext("2d");
//fondo
pincel.fillStyle = "lightgrey";
pincel.fillRect(0,0,600,400);
pincel.fillStyle = "blue"
pincel.beginPath();
pincel.moveTo(50,50);
pincel.lineTo(50,400);
pincel.lineTo(400,400);
pincel.fill();
pincel.fillStyle = "lightgrey"
pincel.beginPath();
pincel.moveTo(100,175);
pincel.lineTo(100,350);
pincel.lineTo(275,350);
pincel.fill();
</script>
