Think about it this way: If you combined X and Y in series, you'd have two parallel resistors. If you removed one of those parallel resistors(Z!), the voltage across and the current through the other parallel resistor does not change.
okay EMF I calculated to be 6V, and then I combined X and Y to make another 3000ohm resistor and because I want output to be 4V, then that would mean current has to change from 2mA to 1.33mA correct? which means i can remove Resistor Z. Is that right im not sure