Because the triangle is an equilateral triangle, you know that all of the angles are equal to 60°. That means that you can use trigonometry to find the height, which would be sin(60) × 20 = 17.32.
Again, because it is an equilateral triangle, all sides are equal as well, meaning that the base is also 20cm. To find the area of the triangle, we must do (20 × 17.32)/2 = 173.2cm^2, which is the area.
