At a given instant, the radii of two concentric circles aree 8cm and 12cm. The radius of the outer circle is increasing at a rate of

and the radius of the inner circle is increasing at a rate of . Find the rate of change of the area enclosed by the two circles.

is this all correct? So I would just add the two da/dt for each circle?

I get

the correct answer is

Can someone tell me where I am going wrong? Thanks.