A spherical balloon is being inflated. Find the rate (in ft2/ft) of increase of the surface area (S = 4?r) with respect to the radius

A spherical balloon is being inflated. Find the rate (in ft2/ft) of increase of the surface area (S = 4?r²) with respect to the radius r when r is each of the following.(a) 2 ft = _______ ft²/ft(b) 3 ft = _______ ft²/ft(c) 7 ft = ________ ft²/ft