The area depends on the radius so we can write A=πr².
Differentiate this wrt time: dA/dt=2πrdr/dt.
dr/dt is the rate of change of the radius=4 (ft/min) while dA/dt is the rate of change of the area.
Therefore, when r=18, dA/dt=2π(18)(4)=144π sq ft/min=452.39 sq ft/min approx.