It's most easily explained if you look at this image (sorry for the bad quality!): 
I haven't drawn it too well, the vertex should be at the y-axis.
I'm not sure how to find the area of the shaded region. The height is variable, and I've got an equation for the parabola based on h. The width of each rectangle is 2.5.
It's very similar to Riemann sums, but horizontal. I thought about using a series, but I'm not sure how.
Any ideas?
Cheers!
EDIT: I should have been more specific. I do know how to find the total area of the shaded region for a specific height, say h=36. However, I'm trying to generalize a function f(h) where I plug in h, and it spits out the total shaded area.
Best Answer
Let the function $y(x)$ be defined: $y(x)=-\frac{h}{36^2}x^2+h$
This function is being restricted to the range of $y(x)\leq 0$. The important piece here is determining when $y(x)$ is zero: $$ \begin{align} y(x)&=0\\ \therefore x&=\pm 36 \end{align} $$ (This was included in your photo.) We can now make specifications on $y(x)$. That is, $y(x)$ is defined on the interval $[-36,36]$. The rectangles will be bounded by two points of the form: $$P_i=(x_i,y(x_i)):y(x_i)=2.5i$$ That is, they will be bounded by two points where the $y$ coordinate (of both points) is a multiple of $2.5$. We can thus see $x_i$ in terms of the $y$ coordinate is: $$\begin{align} y(x_i)&=2.5i\\ \therefore x_i&=\pm\sqrt{1296-\frac{3240i}{h}} \end{align}$$ The length of the $i$th rectangle will be the distance between $(x_i,y(x_i))$ and $(-x_i,y(x_i))$ Since the distance is a horizontal line, the length is simply $|2x_i|$. Thus, we come to the following statement for the total area of the rectangles: $$A(r)=5\sum_{i=1}^{r}|x_i| \quad r=\text{ the number of rectangles}$$ Determining $r$ is essentially the main crux here. I believe involving calculus is where this gets us to. Taking the definite integral with respect to $x$ would give us the total area of the parabola (dependent upon $h$): $$\int_{-36}^{36} y(x) dx=\left[\int y(x) dx\right]^{36}_{-36}=\left[\frac{-h}{3888}x^{3}+hx+C \right]^{36}_{-36}=48h$$ To determine an upperbound on $r$, one must establish the $r$ at which $$A(r)<48h$$ is no longer true. Manipulating this algebraically gives us: $$\frac{|x_1|+\dots+|x_r|}{h}=\frac{1}{h^2}\sum_{i=1}^{r}\sqrt{1296h^2-hi}<9.6$$ So, for a given $h$, you can determine $1$) the upperbound on the amount of rectangles and $2$) the total area of all the rectangles (presuming you use the upperbound as the amount of rectangles).
I can't see how to take this any further, so this is the best answer I think I can provide.