Tangent to the x axis means that the circle touches the X axis
Known information: center (4,9)
The radius of the circle must be 9 units long because the edge of the circle will intersect with the X axis.
standard form of a circle: (x-h)^2 + (y-k)^2 = r^2
where (h,k) are coordinates of center
(x-4)^2 + (y-9)^2 = 81