Well, I am not posting the whole solution.

It'll be better if u try this qun yourself.

I can tell you how to solve this.

Assume co-ordinates A (aCos@,bSin@).

Now, as it's an quilateral triangle,next co-ordinate will be B (aCos(@+120),bSin(@+120))

next will be C(aCos(@+24), bSin(@+240))

Now, you have co-ordinates of equilateral triangle and

I think now you know what to do..

centroid is ((x1+x2+x3)/3,(y1+y2+y3)/3).

Now, it's simple trigo.

still if you have any doubt, you can ask.

Try to do it yourself.