Answer:
The middle slab has 6 cubic feet of cement.
Step-by-step explanation:
In this case we have to make a subtraction to know the quantity of cement used in the middle slab. Mr. Alvarez uses 14 cubic feet total, and the extreme slabs have 4 cubic feet each one, this is:

Now subtraction the total cement and the used in the extreme slabs, we have:

The middle slab has 6 cubic feet of cement.