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.
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.