The triangle it's inscribed in has nothing to do with it. The equation of any circle is (x-h)² + (y-k)² = r² You are given the center as (h,k) = {0,0}, the origin, and you are given the radius r = 2, so (x-0)² + (y-0)² = 2² or x² + y² = 4 Why was the triangle even mentioned? It is totally irrelevant. Edwin