How to do this calculus problem?

Question

The radius of a circle is increasing at a rate of 2 centimeters per minute. Find the rate of change of the area when A) radius=6 centimeters and B)r=24 centimeters

Answer 1

A=pi*r^2

dA/dt = 2*pi*r*dr/dt

dr/dt is given at 2 cm/min

therefore:

when r=6, dA/dt = 2*pi*(6 cm)*(2cm/min) = 24*pi cm^2/min

when r=24, dA/dt = 2*pi*(24 cm)*(2 cm/min) = 96 cm^2/min