The Best Answer for the moment of inertia.
If the side length is 40 cm, then the height is
h = 40cm * √3/2 = 34.6 cm
(from years of looking at equilateral triangles)
The center is 1/3 of the way up the height, so the top is
r = (2/3)*34.6cm = 23 cm
away from the center. That means that all three corners are 23 cm from the center.
Then I = 3mr² = 3 * 0.21kg * (0.23m)² = 0.033 kg·m²
5.2rev/s * 2πrad/rev = 32.7 rad/s, so
KE = ½Iω² = ½ * 0.033kg·m² * (32.7rad/s)² = 18 J