Circle geometry (plus mymaths hate)???

AC is a diameter of the circle, so the centre of the circle is the midpoint of AC. What is $\dfrac{9+1}{2}$? So what did you do wrong?

sorry i don't see, isn't the x difference between C and B 8 and the y difference between B and A 6, so the mid point is 4 along from C and 3 up from B, so basically halfway between CB and halfway between AB squished together?

That's a very convoluted way of saying the same thing, and even if you follow that, you shouldn't get (x-6)² there. The x-coordinate of C is 1, and you're going 4 along from there. What is 1+4?