Application Of Derivatives

By dropping a stone in a quiet lake, a wave in the form of circle is generated. The radius of the circular wave increases at the rate of $2.1 \mathrm{~cm} / \mathrm{sec}$. Then the rate of increase of the enclosed circular region, when the radius of the circular wave is 10 cm , is (Given $\pi=\frac{22}{7}$)