[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}}\\
&0&0&6
\end{array}\\\\
-------------------------------\\\\
(x-0)^2+(y-0)^2=6^2\implies x^2+y^2=36[/tex]
