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Students are not expected to know how to build the additional gates coming into the course. The are expected to look at the truth tables for them, to experiment a bit, to try to find a solution.
The gates should be implemented in the order presented in the book, so the next one you should work on is the Mux (multiplexer). Look at the 'b', 'sel' and 'out' columns in the truth table and see if you can find an And function, then add a 'not sel' column and look at the 'a', 'not sel' and 'out' columns and see if you can find another And. Think about how to combine the two Ands with another function that you already have.
Also, read the book section about canonical representation, that is another way to achieve the Mux function.
You can also search in this forum and on line for more information about multiplexers.
Mark
