Check the picture below.
so then, the distance from the center to that point is really the radius.
[tex]\bf ~~~~~~~~~~~~\textit{distance between 2 points} \\\\ (\stackrel{x_1}{3}~,~\stackrel{y_1}{2})\qquad (\stackrel{x_2}{9}~,~\stackrel{y_2}{3})\qquad \qquad d = \sqrt{( x_2- x_1)^2 + ( y_2- y_1)^2} \\\\\\ \stackrel{radius}{r}=\sqrt{(9-3)^2+(3-2)^2}\implies r=\sqrt{36+1}\implies r=\sqrt{37} \\\\[-0.35em] ~\dotfill[/tex]
[tex]\bf \textit{equation of a circle}\\\\ (x- h)^2+(y- k)^2= r^2 \qquad center~~(\stackrel{3}{ h},\stackrel{2}{ k})\qquad \qquad radius=\stackrel{\sqrt{37}}{ r} \\\\\\ (x-3)^2+(y-2)^2=(\sqrt{37})^2\implies (x-3)^2+(y-2)^2=37[/tex]
