Warning: This might be rough...
First draw it out. Label the angles at the corners of the triangle 60 (definition of equilateral triangles). Now draw a line from the center of the circle to the corner, splitting the corner in half. Label this line R and a corner as 30 degrees. No to find the height of this triangle, you do rsin(30). The base of this triangle is 2rcos(30). Now find the area of this mini triangle (rsin(30)*2rcos(30)/2=r/2*rsqrt(3)/2=r^2sqrt(3)/4). Now multiply this by 3 because you have 3 mini triangles... to get...
r^2 3sqrt(3)/4
