Find x such that point (x,9) is 10 units from (9,3)
All points 10 away from (9,3) lie on a circle of radius 10. That circle will intersect the line y=9 in two places, since the line is only 6 away from (9,3).
(9-x)^2 + (3-9)^2 = 10^2
x = 1 or 17
So, (1,9) and (17,9) are both 10 away from (9,3)