Bom dia,
ficou assim
<canvas width="600" height="400"></canvas>
<script type="text/javascript">
var tela = document.querySelector('canvas');
var pincel = tela.getContext('2d');
pincel.fillStyle = "green";
pincel.fillRect(0,0,600,400);
pincel.fillStyle = "yellow";
pincel.beginPath();
pincel.moveTo(300, 50);
pincel.lineTo(50, 200);
pincel.lineTo(550, 200);
pincel.fill();
pincel.beginPath();
pincel.moveTo(300, 350);
pincel.lineTo(550, 200);
pincel.lineTo(50, 200);
pincel.fill();
pincel.fillStyle = 'darkblue';
pincel.beginPath();
pincel.arc(300,200,100,0,2*3.14);
pincel.fill();
</script>
