I had a question to implement this equation through multiplexers,
f(x,y,z) = Em (1,2,6,7) but the condition is,
only x and y be the selection line,

see the above picture of my truth table, the 3 data line selector multiplexer is reduced to 2 data lime selector multiplexer , but i am confused how to draw its circuit ? because 'z' is now an input.
how to implement 'z' ??

 

You are allowed to use an inverter (NOT gate) to complement the input 'z' and drive it into the MUX. That said, your circuit will be comprised from a 4-to-1 MUX and a NOT gate.

 

You are allowed to use an inverter (NOT gate) to complement the input 'z' and drive it into the MUX. That said, your circuit will be comprised from a 4-to-1 MUX and a NOT gate.

is it so ?

 

Oh, now I see what your problem is.

The Multiplexer is a ready-made IC. You don't have to build it yourself from simpler gates, like you did in post #3. For example, the one you want could very well be this one:
http://www.datasheetcatalog.org/datasheets/134/231447_DS.pdf
You can use it in a schematic just like you drew it in your first post.

The NOT gate I was talking about was the one you have to use to invert the z signal and drive it into the D1 pin of the MUX.