how do u know the centre of the circle is the mid point of A and B the qn only says that A and B are on the circumference of the circle

Yes the question says that A n B are on the circumference. The question also gave you a clue that both A n B lie on a straight line x=5, as they both have the same x-coord. If they do not have the same x-coord, then this method cannot be used. Hence their y-coord midpoint is the y-coord of the centre. A quick sketch of the diagram will do wonders for your understanding and visualisation! Try it out for yourself :) jy

oh ok thank you very much:)