Circles Part 2

Let y=x be the equation of a chord of the circle C1 (in the closed half-plane x≥0 ) of diameter 10 passing through the origin. Let C2 be another circle described on the given chord as its diameter. If the equation of the chord of the circle C2, which passes through the point (2,3) and is farthest from the center of C2, is x+ay+b=0, then a−b is equal to