the plant is at P, which is at 3,5.
now, the plant is right at the center of this closed off region, namely P is the center of the circle, since 12 miles in all directions will make a circle with a radius of 12 units, then,
[tex]\bf \textit{equation of a circle}\\\\
(x-{{ h}})^2+(y-{{ k}})^2={{ r}}^2
\qquad
center~~(\stackrel{3}{{{ h}}},\stackrel{5}{{{ k}}})\qquad \qquad
radius=\stackrel{12}{{{ r}}}\\\\
-------------------------------\\\\
(x-3)^2+(y-5)^2=12^2\implies (x-3)^2+(y-5)^2=144[/tex]