The equation of a circle in radius-center form is



where the center is the point  and
the radius is .

So the equation, all but r, is:



So to find the radius  let's plot the center


 


Now let's draw the line , which is a vertical
line through 5 on the x-axis:

 





Now let's sketch in a circle tangent to that line:

 

Draw a radius from the center (2,1) over to the line.



Using the x-axis as a measuring stick, you can see that
that radius is 3 units long, so



and the final answer is

  or



Edwin