mathematical models(precalculus) problems

Hello i Need Help on mathematical models

11. The surface area of a sphere is a function of its radius. If $\displaystyle r$ centimeters is the radius of a sphere and [tex]A(r)[tex] square centimeters is the surface area, then $\displaystyle A(r) = 4 \pi r^2$. Suppose a balloon maintains the shape of a sphere as it is being inflated so that the radius is changing at a constant rate of 3 centimeters per second. If $\displaystyle f(t)$ centimeters is the radius of the balloon after $\displaystyle t$ seconds, do the following:

a.) Compute $\displaystyle (A \circ f)(t)$ and interpret your result.

b.) Find the surface area of the balloon after 4 seconds.

14. A rectangular garden is to be fenced off with 100ft of fencing material. a.) Find the mathematical model expressing the area of the garden as a function of its length. b.) what is the domain of your function in part a?

c.) By plotting on your a graphics calculator the graph of your function in part (a), estimate to the nearest foot the dimensions of the largest rectangular garden that can be fenced off with the 100ft of material

Thank you very much!!!