not understanding.

A spherical raindrop evaporates at a rate proportional to its surface area. Write a differential equation for the volume of the raindrop as a function of time.

Do i need to use the equations for volume of a sphere and surface area?

I got this:

$\displaystyle \frac{d(4/3)(pi)r^3}{dt} = 4pi(r^2) - \frac{4}{3}pi(r^3)$

I think this is highly wrong tho.