Both point (5,12) and (11,12) lie on the horizontal line y=12.
If these two points is a leg of the triangle, the other leg must be perpendicular to the horizontal line, that is, the other leg must be vertical, but passing through either (5,12) or (11,12), i.e. lies on either the vertical line x=5 or x=11.
There is only one point that passes through x=11, i.e. point A(11,4).