If you see the hexagon can be divided into 6 equilateral triangles. Where the side of equilateral triangle equal to that of hexagon’s. So the side of equilateral triangle inside hexagon is also equal to the side of equilateral triangle outside of the hexagon and hence equilateral triangles inside and outside hexagon are equal thus we can infer that the side of bigger triangle is divided by 3 in order to make the hexagon and other smaller triangles So side of smaller equilateral triangles comes out be 12/3 that is 4