Answer: 
Step-by-step explanation:
Formula: 
Since the base is a circumference, we can replace B for the area of a circumference formula.

What we have here is 12 ft as the diameter of the circumference. A diameter is twice the radius, therefore we can conclude that if we take the diameter and divide it by 2, it will give us the radius.
this is the radius.
Now plug all this information into your formula.

Our values are in feet but the question requires cm. Let's convert from
to
.
Normally, our conversion factors are raised to the power of 1, but in this case it's raised to the power of 3. So, first let's see how many cm are 1 ft.

Here is where magic comes. We can raise both to the power of 3, in order to find the cubic ft and cm that we need as our conversion factors.

Now we have our conversion factors.

Whenever you face the problem that deals with maxima or minima you should keep in mind that minima/maxima of a function is always a point where it's derivative is equal to zero.
To solve your problem we first need to find an equation of net benefits. Net benefits are expressed as a difference between total benefits and total cost. We can denote this function with B(y).
B(y)=b-c
B(y)=100y-18y²
Now that we have a net benefits function we need find it's derivate with respect to y.

Now we must find at which point this function is equal to zero.
0=100-36y
36y=100
y=2.8
Now that we know at which point our function reaches maxima we just plug that number back into our equation for net benefits and we get our answer.
B(2.8)=100(2.8)-18(2.8)²=138.88≈139.
One thing that always helps is to have your function graphed. It will give you a good insight into how your function behaves and allow you to identify minima/maxima points.
You only have the % and you need the number. For example i got a 87% on my Math test what is that as a decimal? The answer is 0.87 because you move the decimal ahead 2 numbers.