Answer:

Step-by-step explanation:
OK, lets start by drawing a basic graph (the first one) so we can visualize.
We already know that the y coordinate of the circle's center is
.
We know that the circle is tangent to the
axis at 
That means the x coordinate of the center has to be
, as the tangent is a point on the edge of the circle that touches a line at exactly one point.
The radius is the distance from the center of the circle to its edge. We know the center's location now, it is
and a point on the edge of the circle (the tangent point) which is
. so the distance between the points is 4 which is the radius (you can use the distance formula, but it's quite oblivious.)
We can imagine the circle should look like this (the second one):
Now we can piece together an equation
The equation of a circle is
where
is the center and
is the radius. When we put the numbers in: we get
which can be simplified into
which is the answer.