[tex]\bf \textit{equation of a circle}\\\\
(x-{{ h}})^2+(y-{{ k}})^2={{ r}}^2
\qquad
\begin{array}{lllll}
center\ (&{{ h}},&{{ k}})\qquad
radius=&{{ r}}
\end{array}\\\\
-------------------------------\\\\
x^2+y^2=25\implies (x-0)^2+(y-0)^2=5^2[/tex]
so notice above, the circle is centered at h,k or 0,0, the origin, and has a ratio of 5.
and y = 2, is just a horizontal line, check the picture below.
