An oil well off the Gulf Coast is​ leaking, with the leak spreading oil over the​ water's surface as a circle. At any time​ t, in​ minutes, after the beginning of the​ leak, the radius of the circular oil slick on the surface is ​r(t)=3t feet. Let ​A(r)=pi r^(2) represent the area of a circle of radius r. Find ​(A*​r)(t).