Answer:
It's a pentagon.
Explanation:
The sum of interior angles in an n-gon is (n-2)*180°
Look at this like this - a triangle's sum of angles is 180°
For each additional vertex you can split it from the polygon by cutting out a triangle. You can split a quadrangle into 2 triangles. You can cut a pentagon into a triangle and a quadrangle (or 3 triangles in total). You can split a hexagon into a pentagon and a triangle... So each additional vertex adds 180° of interior angles.
So we solve for n
540° = (n-2)*180°; divide both sides by 180°
3 = (n - 2) * 1
3 = n - 2
n = 5
So it's a pentagon.