"express V as a function of r" pretty much means make an equation with only V on one side of the equals sign, and r as the only variable on the other.
i.e. V = (2 / 3) * pi * r^3 (functions are usually written "f(r) = (2/3)*pi*r^3")
but NOT
V = (1 / 3) * pi * r^2 * h