We know that the three vertices of the triangle lie on a circle. If we say that point A is the origin (0, 0), then we could say that since the length AC is 3 cm, the co-ordinate of C is (3, 0).

We know that point B lies 2cm from point A making an angle of .

The vertical position of point B can be found by

and the horizontal position of point B can be found by

So point B is at .

Now that you have three points that lie on your circle, you can substitute them into the general equation for a circle and solve them simultaneously for (you are trying to find r).