Hey,

First write down the equation for the enclosed area:

Then take derivative with respect to t, you'll get:

dA/dt = *(r1*dr1/dt-r2*dr2/dt)

Then sub in the values to get dA/dt:

dA/dt = 2*pi*(12*1 - 8*2) = -8*pi

So the rate of change is 8*pi (negative because the enclosed area is decreasing)