Center (0,4) passes through the point (1,4)

(1)   

(h,k) = (0,4) so plug those in:

(2)   

Now all we need is r2

So we plug in (x,y) = (1,4) in (2)

      
      
      
      

So plug in 1 for r2 in (2) and you have
the answer:

      

or you can just write x2 instead of (x-0)2 

        
Edwin