What is the formula for the surface area SA of a sphere having radius r?
When r = 36, what is the surface area?
Note that you are given that dV/dt = -720.
Differentiate the "volume" formula with respect to time "t".
Plug in the known values for dV/dt and r. Solve for dr/dt.
If you get stuck, please reply showing how far you have gotten, starting with the "volume" and "surface area" formulas. Thank you!