It usually helps if, where you can, you draw a diagram. It need only be a sketch.

Do this here, and you should see that your calculation for the mid-point is incorrect.

Add to your diagram two lines parallel with the co-ordinate axes so as to form a right angled triangle and then use the Pythagoras theorem to calculate the length of the hypotenuse (the distance between A and B), and BTW, the answer that you give, 10, is not correct.

The equation that you give for the circle is correct, you have to fill in the co-ordinates of its centre, the mid-point of AB, and its radius, half the length of AB.