check the picture below.
notice the midpoint of the diameter segment, is just 1,9.
also, recall the radius is half the diameter, notice how long the radius is, just 4 units.
[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}}\\
&1&9&4
\end{array} [/tex]
