This is how to do it:
Find the volume of the cylinders: pi*radius^2*height
Find the volume of the rectangular prism: (Subtract the diameters of the cylinders first) Length*width*height
Then, add those together.
What area? It is a 3-dimensional object.
It has a volume. The sides have area. What area do you want?
Find 3/4 of the volume of the cylinders (How to find the volume of it is shown in my answer above), then find the volume of the rectangular prism!