int x1 = 0, y1 = 0, r1 = 1, x2 = 3, y2 = 4, r2 = 1;
Vector2 norm = Vector2.Normalize(new Vector2(x2-x1, y2-y1));
Vector2 v1 = Vector2.Multiply(r1, norm);
Vector2 v2 = Vector2.Multiply(-r2, norm);
Console.WriteLine("(" + (x1+v1.X) +", "+ (y1+v1.Y)+") - (" + (x2+v2.X) +", "+ (y2+v2.Y)+")");